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Here goes all the theories related to the mushroom war.

The Great Mushroom War was the nuclear tear of 2077.
And Adventure Time simply takes place roughly in the 3000s after all the radiation subsided (Of course the Lone Wanderer is dead)

The Lich was contained in the Mushroom Bomb.
And then when it dropped, he possessed an Innocent Bystander.

The Great Mushroom War indirectly put the WHOLE UNIVERSE on drugs.
The Great Mushroom War is widely understood to be a folk memory of mushroom-shaped nuclear explosions. But anyone who watches the show can confirm that it's far too insane for a mere nuclear holocaust to explain, even if the viewer goes through heavy suspension of disbelief. For example:
  • The Cosmic Owl
  • The "glitch" episode
  • That episode where Finn spent his entire life in the Pillow Kingdom before being reborn back in Ooo at the same age
  • Any episode where someone is granted a wish
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  • Other dimensions, like the Nightosphere and Lumpy Space
  • Tree Trunks' deeply unsettling ability to make an apple pie without having any of the ingredients to make an apple pie.
    • ...and I'm sure you can think of more.
      • It's like the nuclear bomb in the first Shin Megami Tensei game; which blew Earth deep into the spirit world. However, in this case humanity didn't survive. What sentience that eventually came about was not human; and thus the old gods, demons, and spirits died and were replaced by those of a non-human perspective.

The Ice King is subconsciously holding back Nuclear Winter.
Look at the opening. We see a dead wreckage of a world; nothing living; with weapons and bombs on a shattered landscape. Then we see the Ice King's domain; then the land of Ooo filled with life. The Crown was powerful enough to to cause a civilization reset event itself when Simon died in the Alternate Universe of the first episode of season 5; the normal universe's Simon used it instead to tame Nuclear Winter and keep this function on "auto" even if his faculties erode.

The Mushroom War took place during the Adventure Time universe's Cold War.
There are a ton of Shout Outs to 80s pop culture in the series, and some of the characters' slang is vaguely reminiscent of the 80s. It wouldn't make sense for Ooo's culture to have been influenced by an era that ended long before our civilization was lost...unless, of course, that was when our civilization was lost. Perhaps, in this universe, the Cold War escalated until someone dropped a bomb, and we all know what happened after that...
  • Looking more plausible after Simon & Marcy. Simon sings the theme song from Cheers while pretending to be the TV. The movie they find on the ground is on VHS rather than DVD or Bluray. The car Simon hides Marcy in is a 70s Nissan. Plus, BMO is pretty 8-bit.
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  • Further supported in the mini-series Stakes where in a flashback (Episode 2:Everything Stays where after the Mushroom War the surviving humans pass along their culture by singing ''According to Our New Arrival'' from the 1985-1990 sitcom Mr. Belvedere.

The Mushroom War takes place during the events of Samurai Jack, and both series have connections with other Cartoon Network shows
Its been established that Samurai Jack, Dexter's Lab, and the Powerpuff Girls all share a universe, and a lot of tropers seem to be under the impression that Fosters and Adventure Time share a universe as well. If one made the assumption the Powerpuff Girls and Fosters also share a universe, then this really isn't hard to see. The Mushroom War was waged by the Nations of the Cartoon Network Universe against whatever nation Aku took control of. Aku was destroyed by the same nuke that caused the hole in the earth in adventure time. The denizens in Samurai Jack who aren't humans are imaginary friends.
  • Or Nickelodeon waged war.

The mushroom war and the magic coming back were two separate events
Magic came back 1000 years ago, but society was able to carry on. The Lich became... well... The Lich, slowly gaining his powers over a couple hundred years, until about 600 years ago. By then his powers were at full capacity, and he tried to take over the world which RESULTED in the mushroom war as a last ditch effort to stop him. It did, but it also plunged the world into chaos, the magic reacting to the nuclear fallout found across the globe. Over the 100s of years before the series, the magic has been affecting and being affected by the nuclear fallout forming the kingdoms we know.This would explain the age of the Ice King vs. Marceline. She may not have been a vampire before, but her father was already pure evil and immortal — these were half-passed to her giving her MUCH slower aging, and a Chaotic Neutral, she most likely became a vampire just to piss off her father, probably around the time she met Ash. This effectively froze her age.The lich didn't immediately attack the world again because he needed the time to rebuild his own body — devastated by the nuclear explosives — and regain his powers.
  • If Marcie was born to the Lord of the Nightosphere, then why isn't she an Apocalypse Maiden?
    • According to the mini-series Stakes she was raised by her human mother and didn't appear to know her father until years into her journey with Simon in the apocalyptic wasteland.
    • The ultimate mindscrew of the viewers would be to setup an entire season on a Prophetic Fallacy. Finn and Jake find an old prophecy about a princess who will destroy the world. After several adventures they find all the signs point to Marcie. At some point they'll have her tied up and because she really really loves Finn then sure, she'll let him kill her, but can he please explain why he needs to? At this point Finn blabs it all out and Marcie reveals that she's already stopped all of this, centuries ago. She kisses him, then tells him that she can never be with him, because he doesn't trust her.

Betty was a member of a science team, and was directly involved in designing the weapons used in the Great Mushroom War.
With small hints throughout the series suggesting she may have been a scientist, as well as suggesting that some of the characters may be her living descendents, it could prove to be an interesting development for such characters to learn of their ancestry, as well as the potential role Betty may have played in such a cataclysmic event.
  • When have there been suggestions about Betty's descendants?
    • For some reason people think Betty and Doctor Princess were the same person until the creators Jossed it. Given the thousand-year difference, if they are connected, it would make sense Dr. Princess is Betty's thousand-year, mutated descendant.
      • We now know she doesn't have any descendants because she took a portal to the future.

There were two apocalyptic events
.According to Pen, the Ice King is 600 years old, and Marceline is at least 1000. In "Holly Jolly Secrets", we learn that the Ice King lived in a seemingly normal, modern city and was a normal human being, 600 years ago. However, in "Memory of a Memory", we see Marceline as a young child, with blue skin and pointed ears (but no bite mark), in the ruined rubble of a modern city. This was at least 1000 years ago. The only conclusion is that after the first apocalypse, which was probably the Great Mushroom War, humanity rebuilt and was decimated a SECOND time, which may have been some cataclysmic event involving the Ice King. This could explain the gunfire and Ice King-like screams in "The Enchiridion"'s outro. Furthermore, the frozen businessmen that appeared in "Business Time", who were some of the first hints at the series' post-apocalyptic story, may have been frozen by the Ice King as part of this second apocalypse.
  • Maybe that place 1000 years ago is their home in the Nightosphere.
  • Maybe the IK froze the businessmen and possibly other humans to save them from the Mushroom War, before he completely lost his mind, meaning the Ice Kingdom could be a completely frozen city under-which the denizens have been preserved for centuries.
    • Meaning Finn could have came from the city. The Hyoomans could hqve been from a part of the city that thawed out.or.escaped.
    • Jossed. IK is actually 40 years older than Marceline, and lived through the Mushroom War.

The Great Mushroom War was fought over literal mushrooms.
When the magic came back, probably due to the reawakening of the Cosmic Owl, it came in the form of new species of mushroom growing all over the world. When they grew in great numbers the land around them became a wellspring of magic, creating the various microcosoms of Ooo- for example, a species of Ice Mushroom grew where he Ice Kingdom now sits, and that's what allows even normal folks like Finn to perform ice-ninja magic there. Localized anomalies, such as talking mountains and superpower-granting mud puddles, also came into being. Those who ate these mushrooms took on their attributes as well, resulting in populations of candy-people, blue warrior people, nut-people, and so on. The war over these mushrooms caused much of normal civilization to collapse, but for the most part society simply mutated into the kingdoms we see in Ooo today. Finn is capable of such heroism despite his lack of magical powers specifically because he's among the last remaining humans. This is also why so many of the show's females are so into him despite his youth, awkwardness, and lack of gainful employment- he can theoretically produce viable offspring with any of them.
  • Somewhat subverted due to Word of God saying that Princess Bubblegum cannot have children.
    • Though, he was probably poking fun at the way the question was worded. They just asked about Finn having babies with bubblegum. They did not put 'Princess' or even capitalize the word bubblegum. Pen was probably referring to the inanimate candy rather than Princess Bubblegum when he answered the question. "if finn had babies with bubblegum would they come out weird?" "I dunno man, I don't think bubblegum can have babies"
  • Alternatively, Finn's ability to pull off so much stuff without even having magical powers isn't because Humans Are Special, but because of Conservation of Ninjutsu—modern-day humans suck so much in comparison because there's just too many of us. This could also explain other things: for example, the victims in most horror movies aren't unrealistically weak/stupid/slow/fragile, they just live in a world with a population in the tens of billions...

Simon Petrikov was indirectly responsible for The Great Mushroom War.
In a relatively short period of time prior to the war, Simon discovered the Enchiridion, a powerful magical book. By the time of the war, he no longer possesses the Enchridion, as he'd given it to a museum. By the time we get to Ooo, the Enchiridion has spent at least some time in the possession of Billy but is under the protection of Mannish Man when Finn goes to retrieve it. Clearly, it's gone walkabout since Simon discovered it. What if one part of its journey brought it into military hands and they used it to reverse-engineer a hybrid magic-nuclear bomb — the Mushroom Bomb?

The Great Mushroom War wasn't fought with nuclear bombs
It was fought with the "C-bombs" from Calvin And Hobbes.To clarify, C-bombs were a throwaway gag in one strip, where they were said to be destructive Fat Man-size projectiles of Calvin's imagination. It stands to reason that a global war fought with these things would result in almost everything gaining life, sentience, and intelligence. Finn and Jake are the reincarnations of Calvin and Hobbes, and share their dynamic. Because of the fact that no other residents of Calvin's hometown have gone on, or at least haven't appeared yet, Finn and Jake don't have any memories of a past life other than being friends and exploring the world around them. However, several people were [[Flanderisation flanderized]] from their pre-war personalities into magical and powerful beings: Marceline was probably an Emo Teen, who was cosmically retconned into being an actual vampire, her dad was a Corrupt Corporate Executive who ordered the C-bombings in the first place, Lumpy Space Princess was a spoiled teen experimenting with drugs, and so on.

tl;dr the bombs were powered by magic and exploded in a cloud of fairy dust instead of boring old radiation.

The Mushroom War was...
A contest to see which country could make the biggest mushroom cloud. As you can see, no one won.

The apocalyptic war was a nuclear one.
I mean...come on. Mushroom War?
  • Supported in that there are nuclear bombs visible among the wreckage in the first landscape that the "camera" passes by in the intro.
  • Combining this with an above theory, couldn't the nuclear war be over the magic coming back?

The apocalyptic war was with the Mushroom Kingdom
Same as above.

The Lich King knows about the history of the Great Mushroom War and was there to see it (When he was originally human)
  • And the Lich King will tell Finn all about it, which leads to...

The Great Mushroom War was fought between humans and Evo Mushrooms.
  • Not only that, everything and everyone in Adventure Time, aside from Finn himself, is an Evo.
    • Dear God, that would be awesome. And that sort of explains the sentient candy.

The Lich was either originally a scientist that built atom bombs or a leader that was part of the Mushroom War
Especially since he wanted plutonium so badly, hates life, and the explosion is much like an atomic explosion.

Necromantic Cyborgs (mentioned above) are reanimated corpses left over from the Great Mushroom War
  • They are all basically human, and "necromantic" implies death in some way, so...
    • Quite an interesting theory there; to expand upon it, it could be possible that they are living people that suffer some sort of mutated version of necrosis that is passed on through genes and increases with age. Thus, they have to replace the rotting tissue with cybernetic parts. This would also explain why Tiffany either has few or no cybernetic parts, whereas some Marauders have several.

The Great Mushroom War was not actually a real war, but the War on Drugs taken Up to Eleven and then failing hard.
Instead of prohibiting illegal substances as intended, drug usage increased to wild proportions. It got to the point where nobody is experiencing reality, but rather everyone is experiencing one huge Mushroom Samba- the Land of Ooo. The war got its name because the primary drug was hallucinogenic mushrooms.

This is theory is just to keep everyone happy.

The Great Mushroom War happened because someone with "wizard eyes" also had an imagination like Jake's.
Meaning, the "crazy crazy crazy" stuff that they hallucinated became real due to their extraordinary imagination, and destroyed the world, recreating it as Ooo.

The Great Mushroom War didn't actually have anything to do with mushrooms.
The name of the war is just what the people of Ooo use, because the only thing they know about the war was the presence of mushrooms (Nuclear Bomb clouds).
  • You don't say.

The Mushroom War was an alternate WWII-Cold War hybrid.
  • So, suppose that nuclear weapons had been simultaneously developed by the Allied and Axis powers around 1943, and that WWII went cold for 60+ years but went hot again some time in the early 21st Century, eventually becoming known as the "Mushroom War." It follows that:
    • Princess Bubblegum was produced by Nazi super-science, accounting for her occasional outbursts in German.
      • Jossed. Princess Bubblegum was spawned from the Mother Gum.
    • Simon Petrikov was a historian investigating Germanic/Norse Paganism under direction from the Nazi Party, which led him to Scandinavia to find the crown that eventually turned him into the Ice King.
    • The Lich was created by a failed attempt to grant immortality or occult powers to Hitler.
    • Actually, pretty much everything weird could be written off as the aftereffects of Stupid Jetpack Hitler or Ghostapo projects causing the magic to come back.
    • Perhaps not coincidentally, the last living human is blond haired and blue eyed...
    • Actually there is post 50's technology in the show, such as Home Media tech.
      • And the theme songs from Mr. Belvedere and Cheers being sung by characters immediately post-war.

The Great Mushroom War was a war of Magic Versus Science
Pen has said that the Land of Ooo is the way it is because of a combination of an apocalyptic war and the return of magic. One notable thing we learn in "Holly Jolly Secrets" is that before discovering the crown, Simon Petrikov didn't believe in the supernatural- meaning magic was at least mostly unheard of at the time. So we have a strong case of magic entering the world, giving a random man super-human powers, driving him mad so he abuses those powers, and all of this happening shortly before a civilization-destroying (probably nuclear) war. Perhaps magic slowly leaked into the world, building over time, creating more and more people like Simon. They would have wreaked havoc on the planet, and the rest of humanity would have united to try to stop them, creating a global war of sane Muggles versus mad wizards. The world's governments eventually used nukes in an act of desperation, inadvertently wiping out most of humanity. But the magic continued to leak into the world, so by the time the rest of the survivors had rebuilt, it had completely reshaped them and the place they lived in became the Ooo we know now.
The Mushroom War took place in the 80s - 90s
From the technology we've seen it doesn't seem very 21st century, or at least mid-to-late noughties.
  • They also have technology that's ahead of the early 21st century (holoprojectors and what-not). However, it's pretty obvious that most, if not all, of what they use is scavenged from the rubble, so it would make sense to have tech from several different eras.
  • It's shown that magic comes while civilization is still intact so perhaps that technology was made using the new found magical materials.

Magic did exist in some form before the Mushroom War.
This would explain things such as the Ice King's crown. At some point in history(likely during the Middle Ages), The Magic Goes Away. However, magical artifacts are unaffected. As to why, it may have been a desperate attempt to stop the Lich's rampage. At some point in his early career, the Lich managed to amass a huge zombie army and began to be a world threat. Without magic, his zombie army fell apart, and he returned to a corpse. Until some scientists brought the magic back...
  • Alternatively, magic artifacts were affected, and the crown was inert until a few minutes after the magic started flowing again; it was simply one of the first things to do so. Simon simply had the bad luck to be the one who owned it when that happened; presumably, its previous owners had put it on without harm. Simon was just unlucky that it had turned back on shortly before he tried it on.

Building from the above, Betty somehow separated into all of the princesses during or after the Great Mushroom War
This could explain why they like the Ice King so much after he shaves his beard (likely making him more recognizable as his true, former self) and why combining them together results in creating a whole new person who treats the Ice King like a husband.
  • Jossed. She comes through a portal to modern-day Ooo to save Simon from the effects of the crown.

In a rare moment of sanity, Simon threw away/gave away the crown to the 'government', but that's exactly what caused the end of the world
The 'government' found it/received it and started to reverse engineer the magic from it in order to make more powerful magic weapons. Instead, they accidentally summoned the previous Lord of the Nightosphere. Instead of fighting, he just wanted to retire. He gave his position to Hunson Abadeer - and made his daughter 1/2 demon so he wouldn't outlive her, in exchange for the plans to make the mutagentic "mushroom bomb" nukes.
  • Parts of this at least have been Jossed Marceline was always half-demon, not made to be so. And Hunson would've outlived her anyway because he is "deathless" while a non-Vampire Marceline would have a (granted) long, but still finite lifespan (As seen in Finn the Human and Jake the Dog). Hunson also calls ruling the Nightosphere the "family business" so if he took over for anyone it would've been one or both of his parents.

The Land of Ooo was once Ireland before The Great Mushroom War
  • I have my own theory where the land of Ooo is located but I still like this one, great Britain was probably nuked into the sea but Ireland could have kept most of its population intact.

After the Great Mushroom War, the Martians helped repopulate the planet/restore order by granting them magic gifts, which would help develop the many weird critters and people on the planet. Lincoln, one of the few human beings left thanks to his immortality, was the one who convinced them to do so, leading to him becoming King of Mars. They may also have been responsible for bringing magic to Earth in the first place, explaining things like the Ice King's crown. This would've added to the whole Mushroom War, as Word of God states that the war was a combination of nuclear weapons and magic. The Martians may have feared such an event would've happened, as their own planet is a desolate wasteland.
  • So is Bravest Warriors (who live on Mars) set before or after Adventure Time?
    • I would have to be after since they appear to be repairing the hole in Earth that clearly hasn't started during the events of the show. This would mean either humans flew around in ships for a while (or found another planet) before some of them come to settle on Mars, or there are some humans living in Mars during the events of AT but are hidden and never seen during the shots of it we've seen.

Magic entered the world before the War.
Things like the crown of the Ice King and other magic objects one day just started appearing. And then a bigger hole opened one day and we got the Lich King. Lich King was never a product of this world but came from a place that was full of magic. He had already taken over and wanted to move his forces into another world to expand his kingdom. All of the nations of the world gathered together to fight him but were no match for the magic that he brought in. The Lich Kings hole in reality also opened hundreds of others letting in things that shouldn't exist but do. Finally humanity figured out they had no chance of winning so the rest of humanity put themselves in cryo sleep and launched every weapon they had at the place where Lich King had opened the gate. The combined explosion destroyed a large chunk of the planet and the resulting nuclear winter destroyed all of the gates. But the magic that was still in the world managed to cancel out any nuclear winter and fixed all of the other problems (such as the fact that the Earth wouldn't be able to function without that massive chunk). Finn was released early from cyro sleep and shot to the surface by a computer error and that is how he ended up as the "last" human on earth.

The Mushroom War was a XK-End of the World Scenario
A very SCP was released, a powerful reality warper, broke free. The world fought against the monstrosity with nukes, killing it and most of humanity. It's abilities were simply spread across the world, which the population assumed was just "magic coming back."

Humans will eventually retake Ooo through force
]]As seen from this fanfic:

The Abraham Lincoln we see in the show was created for the War
Naturally, the material from which he was created was the Statue of the Lincoln Memorial, and he was brought to life and empowered via the recently-returned magic. He was created by the Americans to be a desperately needed leader in time of crisis. Naturally, this implies that the Martians were once human, and are all that remains of America.

The Great Mushroom War wasn't fought with nuclear bombs
It was fought with the "C-bombs" from Calvin and Hobbes.To clarify, C-bombs were a throwaway gag in one strip, where they were said to be destructive Fat Man-size projectiles of Calvin's imagination. It stands to reason that a global war fought with these things would result in almost everything gaining life, sentience, and intelligence. Finn and Jake are the reincarnations of Calvin and Hobbes, and share their dynamic. Because of the fact that no other residents of Calvin's hometown have gone on, or at least haven't appeared yet, Finn and Jake don't have any memories of a past life other than being friends and exploring the world around them. However, several people were [[Flanderization Flanderized]] from their pre-war personalities into magical and powerful beings: Marceline was probably an Emo Teen who was cosmically retconned into being an actual vampire, her dad was a Corrupt Corporate Executive who ordered the C-bombings in the first place, Lumpy Space Princess was a spoiled teen experimenting with drugs, and so on.

The world before the apocalypse was that of Family Guy
In an episode of Family Guy, there was a Cutaway Gag in which the setup is that Peter has gotten a spiritual message before. The clip shows Peter as he claims there is a message in his Alpha Bits cereal saying "Ooo." Brian says they're Cheerios, but actually, they were Alpha Bits and it was a spiritual message. Specifically, one giving a vague hint of the future state of the world. Hey, talking dogs and weird shit exist in both shows, so why not?
  • Wait, Peter was right? Mind blown. Hmmm...maybe some of the Family Guy characters are the ancestors of Adventure Time characters. Jake is definitely Brian's descendant, due to the anthropomorphism and interest in different species. Finn could be descended from Chris, as they share blonde hair.

The Business Men were in some way involved with the Great Mushroom War.
  • Their reaction to Jake asking them what kind of business they did? They try to remember, freak out, and start crying while going "Can't...remember..", They some how were apart of the nuclear war that destroyed the work and, upon remembering, forced it out of their minds, telling themselves that they must never speak of it again, so they literally can't remember or they'll go insane or kill themselves with guilt.
  • Somewhat supported in that Word of God says that the businessmen were the inspiration to the post-apocalyptic setting. If they were the inspiration for the war, it might make sense for their involvement in it to be a part of their back stories.

Related to the WMG above and tying into several others; the Business Men were created by and used to work for Marceline's Dad... who either indirectly or directly caused, or at least was a contributor in, the Mushroom War.
Think about it.
  • Just look at M's Dad and compare him to the BM. They look almost exactly the same (at least until M's Dad goes batshit). M's Dad needed a work force because he happened to be busy, so he created the Business Men to do his work for him. They got out of hand, so he tried to steal their souls, except they had none so he had someone freeze them in a chunk of ice during the Mushroom War.
  • Marceline is old enough to be a survivor of the Mushroom War, and obviously her father is, as well. This speaks for itself; he's an extremely evil guy and he was caught up in stealing people's souls. What's a better way to steal souls and kill people than to cause a war? He seems to be the type of guy to do it anyways.
    • I mean, c'mon; the briefcase and the business suit? He's a great depiction of the stereotypical politician!
  • The song about Marceline's fries can be a metaphor for her losing her innocence, as stated above, but perhaps it's also about her father, with the Mushroom War, destroying all of her hopes, dreams, achievements, friends, etc. and sending the world into chaos. She survived the war and never made up with her dad. Even after they had the exchange of "Ilu Marce!1" "ilu2 dad", she still can't stand to look at him, or at least fears that he'll destroy everything again and acknowledges that his staying in his prison is probably better for the world. She just disguises it as being "emotionally exhausting," which it very well might be as well.

The Ice King caused the great Mushroom war
We now know Simon Petrikov became the ice king before the war, and we know what he's like. Is it really to much of a stretch?
  • Not at all, especially considering the newspaper clipping that Marceline held up in "I Remember You". It's pretty critical and dismissive of Simon's huge discovery. It also seems to imply that he's something of a joke in the antiquarian community. Maybe in combination with the crown feeding him weird visions and bad vibes, he got fed up with nobody taking his work seriously and used the Enchiridion to unleash the Lich and cause Earth's downfall.
    • Similarly, maybe part of the Ice King's gig or even self-punishment is to protect Ooo out of guilt for basically destroying all of mankind.

Alternately, Simon indirectly caused the war by unleashing the Lich.
He had the Enchiridion and did something that accidentally released the Lich. The Lich then proceeded to cause the war himself in an attempt to destroy all life on the planet.

The Ice King caused the conflict in Battlefield 2142 and the Great Mushroom War.
Well, indirectly caused it at least. During Simon Petrikov's video, you can see outside the window that the climate begins to change extremely fast after Simon began wearing the magical crown. By the time of his last recording, the town he's living in and presumably the world are covered in snow. This leads to the situation where world powers begin warring against each other for the few areas on Earth still warm enough to grow crops. Eventually, the war goes nuclear and humanity wipes itself out.
  • Seems to be Jossed, now that recent episodes have shown him to have been mostly still himself even as the war was taking place. It's very likely that the climate change we saw in the tapes was actually a nuclear winter.

Magic began to seep back into the world as the Mushroom War was approaching.
On the Adventure Time earth, magic comes and goes. As the Mushroom War was approaching, magic forces could sense this great disaster coming along. The world could not thrive again without magic, so prior to the Mushroom War it started to return to the world. This is evident in Ice King's crown: it's established that Simon Petrikov lived before the war, likely obtained the crown before the nuclear attacks began. Magic is not predictable, so when suddenly brought into an environment without magic, everything can change drastically.

The crater in the Earth was caused by a meteorite
In Ice King's the "The Past" room, there a picture of a meteorite colliding with the Earth in the same spot the crater is in.

So, after the collision, the human race went to war with itself in order to self-preserve each side of the war. However, the war went nuclear and the rest is history.

The Mushroom War is the Third World War of Star Trek
What explains the Extraterrestrial civilizations?

London lies underneath the Ice Kingdom

My brother actually came up with this theory. While watching 'Holly Jolly Secrets', he pointed out that the buildings in the background of Simon's documentary looked similar to the ones seen in London today.

The Mushroom War is the apocalypse of the Cartoon Network universe
The Litch and Mandy formed an alliance to conquer the world, but the heroes (The Powerpuff Girls, the Kids Next Door, Dexter, Samurai Jack, Ben 10, and others) fought back. Eventually, when the heroes were wiped out, the Litch double-crossed Mandy and allowed her to see the results of her actions before killing her.

The Lich is as old as time itself, and was actually used to power the atomic bombs in the Mushroom War.
Difference is, before being powered up by the explosion, he didn't have even as much as a consciousness, it would explain why the Lich exists exactly the way he is even in Farmworld universe, where large part of the events played differently, the reason to why Finn's wish didn't rewrite The Lich as a whole was because the wish only rewrote The Lich as Finn knew him, or because The Lich was so old that reverting his existence would affect the existence of the multiverse itself.

The Lich is a collective Mind Hive of irradiated souls.
Look at the nuclear blast he's associated with/created-it's a ghastly mashup of thousands of skull-like face mixed into the shape of a mushroom explosion. The Mushroom War became so great that, in trying to create a weapon to end it, they were willing to sacrifice thousands of souls for the ultimate bomb(think the Philsopher's Stone)-likely the souls slain in battle. Said experiments caused the souls to become irreparably insane and omnicidal on accounts of their condition. When the bomb hit the Lich's mortal self, the flurry of nuclear-based, anguished souls utterly broke the poor man's personality and soul. The results were the screaming mania and destructive personality born from the Mind Hive with the intellect and knowledge of the individual who got turned into the Lich, poured together to create the show's Big Bad. There's also the distinct possibility that its inventor transplanted his personality into the weapon, because anyone willing to invent such a weapon is evil enough to be part of what makes the Lich tick.

The birth of Marceline, at least in part, caused the Mushroom War.
The timing does seem to be just right. Marceline's birth may have caused some big dispute that led to the Mushroom War, or cause the release of some magical power. Marceline is stated to be a half-demon, implying her mother was human, or possibly something else. Hunson having a child with a non-demon may have caused some great controversy that led to a war.
  • It has been confirmed that Marceline is half-human. And appears to have been born shortly before the Mushroom War.

The Innocent Bystander who became the Lich was actually trying to stop it, resulting in a failed Heroic Sacrifice.
In the Farmworld Timeline where the regular version of the Lich existed, Simon made a Heroic Sacrifice to stop the bomb. One wonders why he didn't try it in the main universe, however that may be because someone else did so. Given Lich!Jake didn't seem to have the regular version's magical powers(pyrokinesis, telepathy and mind-raping), it could be inferred that a wizard or magical person decided to step in and stop it-why he didn't let Simon do so was because he saw that he'd be important in healing the rest of the world/stopping him if this fails(which proved to be a really good call in "Mortal Recoil.") This backfired, BIG TIME. Of course, this makes the Lich even more horrific—a would-be hero who tried to save the world warped and defected into an undying monster of extreme cunning and malice. Grod help whatever's left of him, especially if there's a lot of it.

  • What if... the Innocent Bystander was related to the Magic Man... what if he, or perhaps, she, was Margle? Or even worse, given that the Lich real-form is a Baby... what if the Lich was their son? Margle (and?) Magic Man tried to stop the Lich Bomb, only to fail, leading to the bomb killing Margle and perhaps possess the clostest, most vunerable living being nearby... their son. Cue [1]

The Lich being an Omnicidal Maniac was a result of oversight by his makers.
]]From what we've seen, the Mushroom Bomb was likely designed to end the Mushroom War. The Lich is the Mushroom Bomb incarnate, and was designed to bring about an end to the war-however they forgot to specify that it was simply the Mushroom War they need ending. When the Lich was born his creators weren't around for this, so the Lich went by his instinct and believed that he was simply supposed to bring an end to war. Since war requires living combatants and there will always be a chance of it so long as there are living things, the Lich decided that getting rid of life would be the most efficent method.

The Mushroom War was fought over the Enchiridion.
First, the Enchiridion is a powerful mystical book that existed before the Mushroom War, and was discovered by Simon. Second, the Lich, who's a product of the war, immediately knows that its a powerful mystical book. And most importantly, the Enchiridion can access a portal to Prismo, a being capable of granting any wish whatsoever and is implied to be GOD. Think of what people would do to get the chance of having any wish come true, especially as these wishes can alter all of creation-which would explain why they felt it necessary to create the Lich.

The dropping of the Mushroom Bomb is what finally broke Simon's mind.
There definitely seems to have been a magitek aspect to it, it was definitely malignant, destructive magic, and Simon's mind was already wearing down by that point. Perhaps the release of all that negative magical energy gave the Crown strength to finally overwhelm him.
  • Jossed: Simon and Marcy happens 996 years ago, while the Mushroom Bomb was dropped 1000 years ago. Though Farmworld!Marceline could've been estimating the time.

The lich is a version of J. Robert Oppenheimer.
For he has become death, the destroyer of worlds"

The Mushroom Bomb was a primitive Genesis Device.
The Genesis Device, as known by Star Trek fans, is capable of terraforming a dead planet. However, it has to destroy the previous biosphere and replace it if used on a living planet. The Mushroom Bomb's mutagenic effects killed off the majority of life on Earth, and mutated what was left into the citizens of Ooo. Since this was created using World War III era Magitek/radiation, it was an irregular and slow progress.

The Lich is voiced by Ron Perlman because...
War, War never changes.Think about it- lots of mutants, post-apocalyptic hellscape and Schitzo Tech? There's even precedent for certain types of magic in the Fallout series. Maybe Ooo is just the Fallout earth much, much further in the future? Or worse, it's divergent from the Fallout timeline because The Master won. And the Litch, being eternal, is the narrator

The Lich is the Mushroom Bomb.
The "fallout" from the Mushroom Bomb definitely looks alive and evil. If so, maybe some of it possessed some unlucky dude and mutated him into The Lich we all know and fear. That's why The Lich is an Omnicidal Maniac — he is a Weapon of Mass Destruction trying to finish the job
  • That's the way it seems to me. The mushroom bomb's purpose was to eradicate life. The bomb infused magic to everything, including itself and its own fallout. The result is that the Lich is the spirit of the bomb, possessing some poor guy's (or girl's.. or dog's.. or snail's) body; working towards the bomb's original purpose.
  • Adventure Time is a kid-friendlier version of Fred Saberhagen's "Empire of the East". Nuclear War causes magic to be asserted in the world, check. One of the bombs becomes a bad guy (the Lich = Orcus) due to the magic, check. Talking dog, check. I haven't read those books in 30 years, but some of the plot flavors are there.
  • Confirmed in Jake the dog. Congratulations, you win a cookie. *gives you a cookie*
The Mushroom Bomb was some kind of Magitek weapon.
As was mentioned further up, the explosion and subsequent fallout from the Mushroom Bomb clearly appear to be alive— not exactly something associated with an ordinary nuclear detonation. Perhaps one of the factions involved in the war, in a fit of desperation, sought to gain the upper hand over their enemy by delving into ancient magical tomes, discovering some kind of powerful spell that could be infused into a weapon and deployed in battle. However, they were messing with forces far beyond their control. They had no idea that once detonated, the Mushroom Bomb's power would physically manifest itself as a being of unspeakable evil— namely, the Lich.
  • It wasn't just a nuclear bomb. It was described as a "mutagenic bomb" or something similar sounding. It mutated everything, even itself. In the new "magic ascendant" Ooo, the most powerful destructive force was no longer a mushroom bomb, but now a lich. This works with the lich's primary motivation being the destruction of all life. He's still a bomb, at heart.

The Lich did not cause the Mushroom War
He did, however, cause it to escalate. Note that bombs were being dropped before he was created, and that the world where he never existed had a distinctly more conventional post-post-apocalyptic feel to it. The war had already started when he was "born", but once he was "born", he changed the course of the war, making it worse and more destructive (and unleashing of magic).

Nekron was used to power the Mushroom Bomb.
Nekron desires nothing less than the destruction of all life. So does the Lich. QED

The Well of Power is the Lich's Soul Jar.
The Lich's Lair appears to be where we was born, given the same dilapidated subway was seen in the Farmworld Timeline with the Mushroom Bomb. It's a power source for the Lich, remains energetic even after 1000 years, and the Lich used it to possess PB when she fell in it. The Well of Power is the exact point on Earth where whoever was hit by the Bomb in this timeline was mutated into the Lich, and acts as his heart. Since the Well is also the one vulnerable spot the Lich has, he made sure to protect it with skeleton guards. If that didn't work, the Lich had a set detonators-the Lair would collapse whenever somebody tried to destroy the well, killing everyone else while simply burying the Well for the Lich to make a better fortress around it later.

Adventure Time takes place in the future of Metro2033
Much like Fallout example above, one can draw a number of parallels between Adventure Time and the Metro universe, most notably the idea of paranormal/"magical" phenomena arising after a nuclear war. The end of the war gave birth to the Lich, and his release is what caused the magic to come back, with things like Anomalies, the River of Fate, and the like being the first results. For a while, there was no afterlife like Khan said, but it eventually returned after civilisation rebuilt itself and returned to the surface. Susan Strong and the Hyoomans (and possibly Finn) are descendants of the last surviving Metro dwellers, and the underground shelter they were found in is what remains of a Metro station (possible Venice, given the level of flooding in it). Also, take note of Ice King's human name— the very Russian-sounding Simon Petrikov. The city he lived in could well have been Moscow, and he and Marceline were only able to survive the poisoned atmosphere of the surface because of the crown and her half-demon lineage, respectively. As for why he was a fan of Cheers (an American show), he probably watched it subtitled on satellite or something.
  • Another thing— notice how the Lich's original lair was a subway station? Yeah...

The release of magic has something to do with the Lich.
In the Farmworld universe, where the Lich never existed, magic did appear to be making some comeback in the Mushroom War era, in small increments. The Ice Crown, and so on. But in Farmworld, there's no magic on any large scale, while there's plenty to go around in Oooo. This suggests that the Lich did something to cause magic to unleash on a huge scale. There are a number of different ways that could've happened...

  • His very presence/creation could've unleashed magic. Maybe the bomb that Simon stopped in Farmworld was magical in nature, and in addition to creating the Lich also radiated more magic to the area.
  • People trying to stop the Lich looked into magic, and did something to release more of it.

Jake got his stretch powers as a result of radiation from The Mushroom War.

The Great Mushroom War occurred on December 21, 2012.
In the Adventure Time continuity, the doomsday predictions came to pass, and the NATO, EU, and Russia fired their full nuclear arsenals at the Axis of Evil, consisting of Iran (which itself consisted of annexed Iraq, Afghanistan, Pakistan, Kazahkstan, and Israel), Syria (which annexed Lebanon, Turkey, and most African nations barring South Africa), and the Korean Empire (which was what North Korea became after annexing South Korea, Japan, China, Mongolia, Vietnam, and the Phillipines). The war started in June of 2012 when North Korea launched a pair of nuclear missiles, one at Seoul and the other at Tokyo. China declared the alliance moot, and Russia declared war. The US also declared war immediately, and Iran and Syria entered the conflict as allies of North Korea. In July, North Korea pushed into the South, forced the US out, and reunified Korea under the Communist banner. Korea then invaded China and, using overwhelming numbers, stolen US and South Korean equipment, and conscripted South Koreans, overran Beijing and forced a surrender when it threatened Shanghai and Hong Kong with recovered American nuclear warheads. China capitulated, and the Koreans turned to Mongolia, which Russia was VERY insistent they defend so they could have a firm buffer against the Korean war machine.

Meanwhile, in the Middle East, Iran invaded and annexed Iraq and made an alliance with the insurgents, promising them a better life in the Iranian Empire that Mahmoud Ahmadinejad dreamed of. Syria, with Korean support, finished off the rebels and Bashar al-Assad set his sights on Europe. The Joint Syrian-Iranian Army (JSIA) ploughed through Turkey and blitzed Poland. Another contingent of the Syrian army, the African Expidentiary Forces (AEF) invaded Lebanon and then pushed deep into Africa, annexing most nations except for South Africa, which held the line in a war of attrition, and Somalia, which Syria considered too rundown and too far lost to even consider redevelopment. Somalian pirate activity rose, but American destroyers finally put a stop to the pirates.

The JSIA had overrun Eastern Germany, and was cutting a swathe through Berlin and along the Rhine. The US launched tactical nuclear weapons and airburst them over the advancing companies and tank columns. Iran responded with a nuclear attack on Jerusalem, and in the hours following the attack, the country was overrun and all civilians in sight killed. Iran immediately tore down the Gaza Strip and recognized Palestine as a legitimate country.

In October, after months of sustained fighting, the Korean Empire had taken over Mongolia, Vietnam, Japan, and the Phillipines. Thousands of civilians were now slaughtered on a daily basis, and everyone was forced to bow down to statues of Kim Jong-un or be killed. The US went on the offensive in the Pacific, successfully liberating Japan and Vietnam, while the Korean Empire went after American possessions in the Pacific, including Midway, Wake Island, Guam, and Hawaii. Korean troops landed in San Francisco on Halloween, the first time foreign troops had invaded America since the Civil War in the 1860s.

The JSIA had taken almost all of Europe, and was now like angry dogs snarling at England. The United Kingdom, though, held out against the incoming air force in the Second London Blitz, and pushed back. A joint US-UK invasion known as D-Day II took place, and most of France had been liberated by the end of the week.

The Korean Empire, now in possession of the most advanced computers in the world, hacked Russian computers when Russia began invading what were once China and Mongolia. The US also invaded the Korean Peninsula, using Japan as a staging ground. Using the hacked computers, Korea, on December 21, 2012, launched every missile in the Russian nuclear arsenal, plus its own missiles, at the United States and Europe. The US, believing Russia had double-crossed them, launch their full arsenal at Russia, Korea, Syria, and Iran. Britain and newly-liberated France launched at Africa, and Iran fired back. Pakistan used the chaos as cover to launch at India. India fired back, and also fired at Korea. 45 minutes later, it was all over. The Northern Hemisphere had been destroyed. The death toll was catastrophic. Over 45% of the human race had died of blast or burn injuries, or were instantly vaporized. The Southern Hemisphere had dodged a bullet, but infighting in Australia over 300 years left Sidney as the last Australian city in existence. The US, Eurasia, and Africa were radioactive wastelands that could no longer sustain life. The survivors tried to celebrate Christmas, but many loved ones had perished, others were dying of radiation sickness, and the presents and Christmas dinners of many had been obliterated.

Enough nukes had been detonated in the Korean Peninsula and Japan to destabilize the tectonic plates. Whoever had survived in that region were killed when that section of the Earth broke away, quite bizarrely, and floated into space. The scientists who had survived couldn't explain the phenomana, and there was no internet left to blog about it.

Several asteroids and meteors impacted Earth over a period of a thousand years. Many were laced with gold, silver, gems, iron, and oil, basically replenishing Earth's resources. An alien ship carrying who is now known as The Lich crashed into Earth, spewing a ton of alien energy that many found they could manipulate, thus explaining where the magic came from.

Ooo is what was once Catalina Island off the coast of California, and sea levels have dropped enough due to the massive crater in the Earth to create the Desert of Doom. Humans have managed to survive, and over 1,000 years, Britain, France, Germany, Spain, Mongolia, and Japan have managed to return to normal after managing to fend off the mutants and zombies created by the magic (Japan had to contend with more than just mutants, though). The Korean Empire has also managed to survive, but as a hollow shell of its former self.

The Mushroom War was part of an Ancient Conspiracy involving wizards.
For a post-apocalyptic world who suffered a war so great a chunk of the planet is missing, Ooo is surprisingly a nice place to live. And it's highly unlikely that the magical craziness of Ooo was the intention of the warmongers. Perhaps the current state of Ooo was intentional. Also, wizards have been known to exist far longer than you'd think: the wizard Bufo has been forcing back a meteor for 847 years, the Ice Crown and Enchiridon are clearly mystical objects long before the Mushroom War, and in "Betty" there was a reference to Antediluvian(aka before the Great Flood) magic. Wizards have existed for millennia. Now why would any Ancient Conspiracy do this? Note how wizards have an influential society in Ooo, and a definitive power base. Though there were other reasons for the Mushroom War, at the very least the specific mutagenic weapons were due to a conspiracy set up by some wizards in order to assume power over the world. Given that this war led to the Lich, it seems to have been too successful.

There will be a made-for TV Movie dealing with the Mushroom War itself.
It's way too big a story to be told in one episode or even a two-parter. The Mushroom War will be part of a movie split into five standard-length episodes. Part One will deal with the initial build-up, exploring how magical artefacts(eg Simon's crown) existed at all. Part Two will deal with the causes behind what started the Mushroom War. Part Three will show how it became so devastating, ending in the birth of the Lich. Part Four deals with the Lich's first attacks and the aftermath. Finally, Part Five will deal with how the world heals and Ooo is born, ending with the origin of the Candy Kingdom.

Something Manipulated the Mushroom war to happen and return magic to the world, that thing is what became the Lich
The Lich was born from the Mutagen Bomb, however in the modern sense a Lich comes from someone purposefully casting the spell to turn themselves into one. So I propose that this person, perhaps the spirit of a mage from when before magic left manipulated events to do the one thing that could bring him back to life, by using a mutagentic bomb to hit one of the magical leylines which had been lying dormant, giving magic a kick start and allowing the spirit to possess one of the victims, and at the same time binding the magical leylines of the world as his phylactery. As a secondary guess to this, the spirit that set this all up died trying to lichify himself using the leylines before, and was stopped by a powerful ice mage, who sealed his spirit in a crown just in case.

The Enchiridion led to the mass release of magic on Earth.
Someone activated it soon after it was found, opening a portal to the multiverse (and possibly portals to other universe) and causing magic to rush in.

The pink goo was some war-related experiment.
Either a weapon or an attempt at protection. Whatever it was might not have worked as intended. Alternately, it could've been an attempt at creating more resilient life forms who could survive in the wasteland and continue society.

The human that became/was inhabited by the Lich
with the Mushroom Bomb? It's the Ice King's son.Recent events have the the Lich being properly resurrected, and as a result becomes an infant. The Guardian Blood didn't turn Finn's dad's bony leg into a baby leg, so the person hit by the bomb was probably a baby. By all rights Simon should've tried to save the world in the main timeline, so the person who saved the planet would need to have an impact on him. What if that child is his? Yeah, the crown is driving him crazy, but asides from that he's a lovable guy. Simon could've found love again, but it failed miserably-his new love left him, and gained custody of the baby.

The Mushroom War began with an Alien Space Bats version of Reagan's "Star Wars" program.
For those not in the know, during the 1980s President Ronald Reagan suggested a solution to the possibility of the Soviets sending nuclear bombs: the Strategic Defence Initiative or "Star Wars" project, effectively neutralizing nukes before they can hit. One such example suggests using space-stations to eliminate nukes from space. It didn't happen in Real Life because it's too difficult to pull off with 80s tech. However in the world of Adventure Time, magic exists(at the very least magical artifacts given the Ice Crown's existence.) Using this, the out-there plans Reagan had to combat the Soviet Union were used. Unfortunately this didn't stop the war, but rather motivate the Soviet Union to up the ante. Eventually this led to the Cold War finally heating up into the Mushroom War. Note that this isn't meant to be a criticism of Reagan's policies.

The Mushroom Bomb was a summoning ritual.
The Lich existed long before the bomb went off, as a sort of death spirit. Whether he was always present or born out our self-destructive tendencies is unclear, but existed as a sentient being before the bomb went off. However, the Lich could never enter into the physical plane until it went off-maybe Death kept the Lich restrained, which explains why he was creeped out by him. Presumably they used the Enchiridion.

Evangelion: Final:|| concludes with the magic returning to the world
Assuming WMGs about Rebuild of Evangelion being a sequel to Neon Genesis Evangelion are correct, it's entirely possible that the series concludes with the magic returning to the world, and somebody just makes up the great mushroom war as an excuse for the devastation that occurred before.

The Mushroom War was fought over the little magic that existed at the time.
It's been shown that magic did exist prior to the war, but apparently not in large quantities. But even the magic that was available could be very powerful, and the War was fought over whatever magical artifacts the warring parties could find.

The nuclear bomb was the one that bought all the monsters. And the comet (that was shown in one of Finns past lives)was a magic comet that bought all the magic.
And the comets hit the nuke at JUST the right time.

The nuke was cursed.
This explains how when the nuke exploded in the Lich, the clouds of radiation that appeared from the bomb, looked like souls.

Adventure Time takes place in the same world as My Little Pony: Friendship is Magic.
Because why the hell not?

The giant creater on Earth which is sometimes shown,
Is actually a giant portal in disguise. Not even Prismo knows about this.

The Mushroom Bomb war was fired by the Americans.
We know that the Lich existed in some form way back in the age of the dinosaurs, as the comet that did them in. However since Prismo's wishes generally work(even with an ironic twist), we know that he didn't exist as a lich until the Mushroom War-so he was probably stuck in the crater he landed in(though that doesn't discount the idea he manifested in non-Lich forms before). Said dinosaur-killing crater would be in the Gulf of Mexico, where America could easily find it. During the Mushroom War they had the opportunity to mine the radioactive gloop that was the Lich, and packet it to be fired at their enemy in the Mushroom War. It failed spectacularly, and there's every chance the Lich goaded them into using him. Going back further, knowledge of his essence may have led to the very first atomic weapons being made.

The crater wasn't created by a bomb.
It was created when the Cluster hatched.

The Mushroom War was fought over a comet.
We know that a catalyst comet hits Earth every 1000 years, bringing an agent of change. As such, said comet would be around the same time as the Mushroom War. Here are a few possibilities of what might have happened:

mushroom is the 5th element
the crater is where the orginal mushroom core was. the inhabitants managed to create a crude mushroom bomb, which blew up the core.
  • The Mushroom Bomb was based on lumps? Huh...I guess that explained why Sweet P wasn't affected by the transformation.

Homestar Runner takes place after the Mushroom War
One of the most common Homestar Runner WMG's is that the cartoons take place after some sort of apocalypse that caused the characters to mutate. Given that the Mushroom War caused any number of similar mutations, why not combine the two?

Honestly, there's not a single Homestar Runner character who wouldn't fit right into Adventure Time if the art style were slightly adjusted. Homestar Runner takes place in what's left of North America, the so-called "Free Country, USA," while the Land of Ooo is Europe. While the few maps of Ooo that we see are unrecognizable, some of the colonists from Islands (who left Ooo in Stakes) spoke a Nordic language, implying that Northern Europe was part of the landmass that became Ooo.


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