In Real Life, wars are a messy and tiresome affair. Hostilities can escalate for months long before the sides even exchange the first shots. Even the best war machine in the world will still take its time securing the enemy lands, trench by trench, if not because of fierce resistance on both sides of the frontlines, then because marching in too fast will stretch the lines of communications and supply too thin for normal operations.
In fiction, however, the dramatic rules require the invaders to appear larger than life, to establish them as a credible threat unbeatable by normal means. The simplest way to emphasize the superiority of the invader's military might and technology is to establish that they completed their takeover before the invaded state even knew they were there. The survivors will later refer to the invasion as the "One Week War", or the like.
Alien Invasions often receive this treatment but it is not limited to them; for instance, rogue AIs usually take nanoseconds of processing time to peg humans as the enemy and launch a Robot War. However, it is almost always some fantastic element that allows for this trope to occur and bypass the standard logistical problems.
A large-scale subtrope of Curb-Stomp Battle. Often involves Easy Logistics. A common setup for Back from the Brink scenario. Home by Christmas would be when someone is expecting this kind of war, only for things to turn out much less than expected. Compare also Easily Conquered World, wherein the defenders fail to offer any resistance to much weaker invaders.
Note that the title shouldn't be understood literally: to qualify, conquests don't have to take place within a single night; any improbably quick conquest (relative to the size of captured territory) qualifies.
- This is pretty how the Kushan Empire invaded the Capital City of Windham in Berserk. Moments after the King of Midland dies, Emperor Ganishka and his troops come storming in and take over the joint.
- In Mobile Suit Gundam F91 the Crossbone Vanguards quickly take over the colony within mere hours, as forces of The Federation were borderline weak to useless in defending the colony.
- Marvel Comics has a 2012 Crisis Crossover called "It's Coming" which deals with a Bad Future in which The Phoenix is supposed to come back to Earth and reduce it to an ash-filled wasteland devoid of life. In the future, they call the cataclysmic event "The Six-Second War".
- In We Stand on Guard, the United States invades Canada after The White House is destroyed. Ottawa is flattened in an instant, and the rest of the Canadian Forces are routed within the first week, allowing for a swift occupation.
- Stargazer has the Cabal's invasion of Ceunon. A werecat who witnessed the battle firsthand claims that Ceunon officially surrendered within one hour of the siege while the Cabal took zero casualties. It then only takes a single day for the Cabal to stomp out any pockets of resistance and establish total control over every aspect of life in the city. Since Ceunon is a medieval castle town while the Cabal are an alien army with access to advanced technologies like power armor, hover tanks, and drop ships, this is hardly unbelievable.
- In A Thing of Vikings, the military use of dragons completely upends the existing military paradigms of the day. City walls become something that keep defenders from escaping, as opposed to keeping dragon-riding attackers out. Dragons make it so easy to conquer that Berk with a relatively small force conquered Vedrarfjord by accident when they only wanted to get rid of the king. When they do it on purpose, Harthacnut's reign over England and Denmark ended in short order.
- In Canadian Bacon the Canadian-American war ends before hostilities can even begin when the Americans realize that all their nuclear weapons can be remotely controlled from Toronto. The American President surrenders to the Canadian Prime Minister who is not even aware that anything out of the ordinary is happening.
- Red Dawn (1984) has the Russians occupying a large swathe of the USA within the space of days.
- Red Dawn (2012) does the same, replacing the Russians with the Chine— sorry, North Koreans.
- The Psychlos of Battlefield Earth beat down humanity in about nine minutes. Presumably all the mighty warriors responsible have since left to conquer other galaxies and left the dregs on this backwater; it's the only possible explanation.
- The Psychlos don't fight in the conventional sense. They teleported nerve gas drones all over the world. Presumably, human forces didn't even have a chance to fight.
- The Chronicles of Riddick has Helion Prime taken over by the Necromongers in one night. Then again, we're only shown a single city, but the Necros treat it as if the entire planet is under their control.
- Robert A. Heinlein's novel Sixth Column (AKA The Day After Tomorrow). The PanAsians use their vortex beams and A-bomb rockets to defeat the United States in less than a day.
- The Mouse That Roared: The Micronation of Grand Fenwick declares war on the US with the premise that they will lose and then get repaired, pumping money into the Fenwickian economy. Then they accidentally capture the Q-Bomb and win. This is before anyone in the US even knows the war is going on.
- At the beginning of The Ellimist Chronicles, Toomin and his friend are playing a game. Toomin's side is defeated so quickly he has to watch the replay in slow motion to find out what happened. Of course, given the grand scale of the gamenote , this might not be saying much.
- In Michael Moorcock's "Eternal Champion" story, the Eldren, under their military commander former human Champion Ekrose kill every human on Earth within a very short (unspecified) span of time. Justified as the Eldren have ray guns, and the humans have classic middle ages armor, swords, and so forth.
- Cain's Last Stand makes a very big deal that an invading Chaos force is progressing far faster than it has any right to, and even seems to be growing. They find out that it's because Warmaster Varan has the ability to instantly brainwash any who hear or see him, including over a public-address system. In addition, Necrons have weapons that disintegrate targets and can teleport to just about anywhere they please, resulting in a Curb-Stomp Battle.
- The prequel book of The Tripods indicates it only took a few weeks after the Trippies started passing out Caps for the world to end up with more Capped than Uncapped. Some pockets of fighting went on for a while, but it's obvious by the end of the prequel that the Capped are in control. Downplayed by the fact that while the actual fighting takes a relatively short time, the invaders had clearly been quietly laying the groundwork for a considerable amount of time before the first shot was fired.
- The pirates in Invasion of Kzarch conquer the planet, more or less (A few holds-outs exist.), in just a few days. Justified, really; the planet's military isn't very good, and the pirates had near complete surprise, control of the air, and out-numbered the army substantially.
- In the Drake Maijstral series, the alien race called the Khosali showed up and surrounded Earth with a hundred thousand warships at a time when the Earth had only a handful of interplanetary ships. A few hundred Earthlings on military stations put up some token resistance, but they were quickly defeated, and Earth had no choice but to surrender completely.
- Battlestar Galactica (2003) begins with the near-obliteration of the human race in a surprise attack that's over before anyone's realized what's going on.
- Enemy at the Door opens with the Germans occupying the Channel Island of Guernsey in World War II. The Germans basically just walk in and take over before the news of their arrival has made it all the way around the island. This is Truth in Television, and less a sign of the Germans' superiority than a reflection of the fact that the islands were entirely defenseless; the British High Command had decided that trying to fight the Germans off would cause a lot of collateral damage to no worthwhile effect, and had withdrawn their entire military presence from the islands a few weeks before the Germans showed up.
- Between the first incursion in Half-Life and the thoroughly subjugated world of Half-Life 2 is "The Seven-Hour War".
- In the Strangereal continuity of the Ace Combat series, the Belkan War began on March 25, 1995 and would have ended with Belka's victory on April 2, if it were not for the single remaining ace (that's you) who delivered his country Back from the Brink.
- The Reaper invasion of Earth in Mass Effect 3. As lampshaded in the intro sequence, it took them minutes to cut through all the defenses piled up around the human homeworld and land in force. There remains a small planetside resistance but Earth is otherwise firmly under Reaper control for most of the game.
- The Batarian homeworld fell even quicker, as many of their leaders turned out to have been indoctrinated and disabled the defenses.
- The asari homeworld Thessia also falls fairly quickly once the Reapers themselves get involved; it has a longer resistance to husks, many which die instantly to any directed force attack, because the entire asari population is biotic. Of course, the Reapers are devastating forces of destruction, they're there en masse, and asari culture isn't very military-oriented to begin with; the turian homeworld Palaven, held by a race who consider military service to be a Rite of Passage and get krogan support at the end of the first act, is a comprehensive aversion even though when you first see it most of its major population centres are on fire. Kahje, meanwhile, will be either perfectly safe or instantly doomed depending on your solution to the notoriously buggy Citadel: Hanar Diplomat mission.
- In Sunrider, the PACT invasion of Cera begins and ends with the PACT flagship Legion coming out of warp, obliterating most of the Cera Space Force with a single salvo, and nuking the planet’s capital city without giving them a chance to surrender.
- The Warcraft franchise has this problem with its wars in recent canon. Originally, the First War was preceded by ten years of border skirmishes with the orcs before it became an all-out war that lasted about five years. The lead up to the war was eventually retconned away, leaving a simple, five-year war. The gap between the first two wars used to be six years, before later being changed to just two, with the war itself only being about two years. The original third war seems to have been retconned out entirely in favor of a single battle, with the events of Warcraft 3 now taking the title. It's almost as if more territory makes a war take less time in the Warcraft universe.
- This has gotten even worse starting with World of Warcraft, with every expansion being said to take place over the course of a year (with a one year skip after Cataclysm), no matter what the expansion is about. Wrath of the Lich King, for instance, involved an all-out war against a world-spanning undead army, and somehow was still resolved within a year (made even worse when you consider that the Undead Scourge survived several major blows across two years during the Third War when they were a relatively new army and weren't as entrenched as they'd be during Wrath of the Lich King).
- World War II: The German 1940 invasion of Denmark is successful, with the Danes capitulating within six hours. The invasion of France in the same year sees German troops parading through the streets of Paris less than two months later, compared to more than four years of conflict with France in the First World War.
- Thunder Cats 2011: The Lizards have devastated the entire kingdom of the Cats in just one night. Justified as the Cats have medieval level technology, while the Lizards are armed with high tech weapons such as laser guns, bombs, and giant mechs.
- The early stages of World War II.
- Germany conquered the Netherlands and Belgium in a matter of days.
- Then there was Denmark, when the Germans mounted a surprise invasion on the morning of April 9, 1940, which led to Denmark's surrender in about six hours.
- Iraq vs. Kuwait
- Iraq, then under the rule of Saddam Hussein, invaded and occupied Kuwait in just one day in 1990. It helps that Kuwait was a small country and its military was at a size of about 16,000 strong. This pales in comparison to Iraq which was a fairly large country with a military of about one million men (the fourth largest army in the world at the time)!
- Then it got inverted with the liberation of Kuwait by the U.S. led Coalition. While the air campaign went on for over a month, the ground campaign lasted just 100 hours.
- Argentina's invasion of the Falkland Islands on April 2, 1982, also qualifies. It helps that the islands are sparsely populated and that the British only had about 57 Royal Marines and some assorted sailors and civilian volunteers to defend the islands at the time. In spite of this, the Brits actually managed to force the initial Argentine assault force to retreat for a bit, and Governor Rex Hunt initially refused to surrender. Only after the Argentines brought in more troops and surrounded Government House did a surrender occur. This, of course, touched off The Falklands War. The Brits reclaimed the islands within two months.
- The Conch Republic in Florida. In 1982, after the Border Patrol set up a checkpoint at the only road out, Key West declared independence and attacked the United States... by whopping a man in a Navy uniform over the head with a loaf of Cuban bread. One minute later, they surrendered (to the man in the Navy uniform) and applied for one billion dollars in foreign aid.
- There were a couple of real wars with names like these, such as Six-Day War where Israel had a major victory over several Arab nations in a course of one incomplete week in 1967, or Russian-Georgian war in South Ossetia in 2008, which pretty much turned into a full-scale Georgian rout after its second day, but is commonly called the Five-Day War, because of the cease-fire agreement date.
- There's also the Anglo-Zanzibar War, which lasted all of a whopping 40 minutes before the conflict ended. It's the shortest war in history.