Read or Die: The leader of the I-jin has revealed himself, announced his Apocalypse How and revealed Nancy is Mata Hari and spying for them. In response, Gentleman informs Joker over the PA that Operation Manuscript Retrieval is suspended...
Joker: British Library Special Engineer Force, suspend Operation Manuscript Retrieval immediately. As of now, we begin "Operation Exterminate I-Jin Corps!" (cheers)
Also a Crowning Moment Of Awesome, Luffy and the crew (Mostly Sogeking) from One Piece declare war on the World Government by not only shooting the Government Flag, but burning it as well. As it stands atop a major governmental base. Being watched by hundreds of marines.
Later, Admiral Kizaru is fighting with four of the most powerful rookie pirates in the world. He's clearly toying with them, but doesn't go for the finishing blow. Then one of them hits him with a surprise attack that cuts him in half and blows the pieces to smithereens. After he pulls himself back together, he's clearly had enough. He then declares the fight is over and dispatches them all in a matter of seconds.
Luffy has now declared war on one of the Four Emperors, Big Mom.
In Berserk, Guts gives a Rousing Speech variant of this against The Apostles and The Godhand after surviving The Eclipse.
Volume 3 of Hellsing has Millenium essentially declare war on Hellsing when they openly send one of their operatives, backed up by troops from the local law enforcement, to attack Alucard and Seras, live on national television no less. Just in case they didn't get the message, Volume 4 has them send a messanger to Hellsing headquarters with a live link to the Major, so that he can pretty much say "Let's fight!".
The homunculi of Fullmetal Alchemist caused major problems for themselves because of this trope. Before Maes Hughes' death, Mustang knew pretty much nothing about the homunculi and consequently was not involved in trying to stop their evil schemes. The moment one of them killed Hughes, he quietly went into a sort of chronic Heroic BSOD mode and became such a danger to them that Lust finally decided he wasn't worth it as a sacrifice and simply tried to kill him.
In Naruto, Tobi declares war on the entire ninja world after the Kages refusal to hand over Naruto and Bee, starting the fourth great ninja war.
Played for drama in the Astro City story "Serpent's Teeth", when an alternate-timeline version of Jack-in-the-Box's son uses Jack's "Of course you realize, this means war" as motivation to become a Knight Templar on the city's criminals... without realizing Jack was quoting Bugs Bunny.
In The Smurfs comic book story "The Smurf Threat", the Grey Smurfs send an Arrowgram to the Smurfs, warning them to surrender Hefty or else. Papa Smurf refuses to turn over Hefty to the Grey Smurfs, and the next Arrowgram reads something to the effect of "this means war" — with the next thing the Smurfs see is the Grey Smurfs surrounding the village.
Serenity: "So no more running. I aim to misbehave."
Wrong Is Right: The US president uses this reason to invade the Middle East after 2 nuclear warheads are found in New York City. However, the film is a parody of Patriotic Fervor and Eagleland as the warheads were allowed into the city to ensure the president's re-election.
Layla, one of the characters in the superhero film Sky High, won't use her powers except in self-defense, which means she gets relegated to the Sidekick track, constantly teased, and thought of as a complete wimp. Then one of the bad guys, convinced that their side is about to be victorious and not being able to resist, slaps her. "Big. Mistake."
"This means war, gentlemen! Our passage to India is threatened, I should think, wouldn't you? The honour, the reputation, the glory of England is threatened. Our Queen's Majesty is sure to be threatened, she is... If the Turks go down like cards, flip-flop, then next up our own Solent, against our own Queen will come the Russians - ships and guns to rip our country into shame.
The novel Point of Impact, made into a movie called Shooter, had Bob Lee Swagger get upset when they framed him for trying to kill the President and tried to kill him. But he really declared war when they killed his dog!
Happens a couple other times too. Molly getting dragged into Arctis Tor, the Nickelheads kidnapping Ivy, one side story when Michael's daughter is nearly killed, Bianca after Susan was turned... Let's face it. If you truly piss off Harry, stick you head between your knees and kiss your sorry ass goodbye.
Changes: he pretty much goes to what his threat was.
Though not in the literal sense, Anne Shirley, declares war on Gilbert Blythe after he teases her for her red hair by calling her "Carrots". She becomes his academic rival, and vows that she will always beat him and never forgive him. She eventually does forgive him, and goes one step further by marrying him.
In the appendices to The Lord of the Rings, the section Of The Dwarves recounts how Thorin's grandfather went to Moria and was caught by orcs, who threw out his head with a purse of small coins stuffed in the mouth. The purse was taken to the Dwarf-king Nain who regarded it in silence for a day or two, then said "This cannot be borne". The dwarves suffered horribly in the years that followed, but the orcs of the Misty Mountains took a hiding.
In Mission of Honor, an entire treecat clan is wiped out during Oyster Bay. In A Rising Thunder it turns out that one member of the clan was visiting a neighboring clan and survived... and now the treecats have declared war on Mesa. For the uninitiated, a treecat is a telepathic, six-limbed, furry buzzsaw of DOOM.
Fattypuffs & Thinifers:
At that moment a soldier entered bearing a message from President Rugifer. It said that the proposal which he had sent by his secretary was the last he would make, that it was an ultimatum, that he gave the Fattypuffs ten minutes to accept it and that he had ordered his train and would leave immediately if his wishes were not complied with. "This means war!" said Marshal Puff in despair. "C'est la guerre!" repeated Professor Ramfatty with satisfaction.
Doctor Who: The 10th Doctor in "The Idiot's Lantern".
The Doctor: The street. They left her in the street. They took her face, and just chucked her out and left her in the street. And as a result, that makes things... simple. Very very simple. Do you know why? Inspector Bishop: No. The Doctor: Because now, Detective Inspector Bishop, there is no power on this earth that can stop me!
This a trait that all Doctor's can show pieces of. Although they rarely use the word 'war', if you hurt anyone close to them, anyone they love, you will pay.
The title doesn't just describe the Doctor, but also Rory. After the Silence kidnapped Amy and his unborn baby, Rory immediately goes on the war-path, even going so far as to storm through a Cyberman warship and scaring the crap out of them for information.
Cyberman: Daleks, be warned. You have declared war upon the Cybermen. Dalek Sec: This is not war! THIS IS PEST CONTROL!
On Good Eats, host Alton Brown, in response to one of the Mad French Chef's declarations, said to the camera, "I hope he knows, this means war!" in a fake French accent.
He also declares war on a cupcake shop that's run like a fancy restaurant, complete with a maitre'd taking reservations out front, saying that they're sucking all the joy out of cupcakes.
Played with on Supernatural, when Dean tells Rufus, "It's war, you hear me? It's war!" The latter replies with, "You're damn right it is," before punching Dean in the face, not realizing that he actually meant War.
In Gossip Girls fourth season Chuck declares war on Blair after she drives Eva away, though one might wonder where he feels entitled to declare war over something like that after he sold Blair for a hotel and slept with her nemesis in the previous season. Chuck and Blair are then at war for a few episodes until Nate and Serena sit them down and write an actual legally binding peace treaty. These kids sure do take things seriously...
On Boy Meets World, Rachel actually says these words after she fails to get Cory and Shawn in trouble for a prank they pulled on her. This leads to an Escalating War.
"This is the Commander. Moments ago, this ship received word of a Cylon attack against our homeworlds is under way. We do not know the size or the disposition or the strength of the enemy forces, but all indications point to a massive assault against Colonial defenses. Admiral Nagala has taken personal command of the Fleet aboard the battlestar Atlantia following the complete destruction of Picon Fleet Headquarters in the first wave of the attacks. "How? Why?" doesn't really matter now. What does matter is that as of this moment, we are at war. You've trained for this. You're ready for this. Stand to your duties, trust your fellow shipmates, and we'll all get through this. Further updates as we get them. Thank you."
Attack Attack! (the US band) released an album in 2012 titled "This Means War."
Petra's 1987 album and single, "This Means War!"
In Traveller, the titular Travellers make sure to not do anything that louses up the Imperium's internal economic and political cohesion. Small wars between sub-states are ok; after all boys will be boys. But if you extend your war too far and bother your neighbors, use Weapons of Mass Destruction or otherwise become too much of a trouble then you will receive a visit from the Space Marines.
Subverted in The Godfather game. Shopkeepers and racket bosses will fight back if you apply too much pressure on them and you won't be able to get a payout from them... but since your pressure-application would probably have involved hitting them, shooting them, harming or killing innocents, all this does is mark them as Too Dumb to Live. Sure, you'll need to come back in a week's in-game time to try again should you kill them, but really, they knew what would ensue...
Played for Laughs in the Mass Effect 3 Citadel DLC: By this stage a lot has been done to Shepard, and the plot is someone is trying to kill her/him, a Cerberus clone. S/he brushes off numerous insults, Kick the Dog moments and attempts on her/his life until the clone tries to dispose of Shepard's pet space hamster. S/he reacts the same as s/he does in regards to Batarian slavers, finding David Archer, and the renegade resolution to Tali's trial. Combined.
Shepard: They messed with my hamster, guys. Now it's personal.
In Dragon Age II, Meredith's decision to issue the right of annulment on the Kirkwall Circle, despite having nothing to do with Anders' destruction of the Chantry causes the Mages in Kirkwall to finally have enough, with their rebellion either supported or quashed by Hawke. The events at Kirkwall later ripple across the face of Thedas and lead to other Circles rising up as a result, leading to the outbreak of open war, three years later.
In Saints Row: The Third, Monica Hughes almost says this (her exact words are "this is war") when Killbane has the Hughes bridge destroyed.
At the climax of The Cartoon Man, Simon pushes Roy a bit too far, causing him to say the line, "You do realize of course that this means war!" and begin an over-the-top Looney Tunes-style cartoon battle.
Looney Tunes: A Bugs BunnyCatchphrase of course ("Of course you realise, dis means war!") — any time Elmer Fudd, Yosemite Sam or some other buffoon harasses him to this point.
Bugs Bunny's Tiny Toon Adventures counterpart Buster Bunny would say "It's time to party - Buster-style!" in the same kind of situation that would prompt Bugs declaring war on whoever's provoked his ire.
The Animaniacs episode "King Yakko" has the bad guy say "this means war" when his first attempt to take over Anvilania fails. However, he was holding his hands over his head when he said it, leading to Yakko's rejoinder:
Yakko: "I thought that meant 'touchdown'."
Slappy Squirrel (who is a clear expy of the likes of Screwy Squirrel, Bugs Bunny and the may other characters who embodied this trope in the golden age of animation) will make your life hell if you cross her, even if it's just for petty issues. Just ask the neighbor she drove to insanity because said neighbor wouldn't let her throw away a soda can in her trash.
Slappy: If I was a better person, I'd ignore this and move on with my life. But I'm not!
In one episode, Lois gets splashed on by a car driven by her rival, who constantly beats her for the award for best piano student. Needless to say, This Means War.
In another episode, a bomb blows up in Meg's face, which leads to her suddenly having a duck bill backwards on her head... exactly like what happens to Daffy Duck in Looney Tunes. She then says, "Of course you realize, this means war!".
Brian in "Halloween on Spooner Street", when he tried to reason with the bullies who took Stewie's candy, only for them to dye his fur pink.
In DuckTales, Fenton Crackshell (prior to his becoming Gizmoduck) declares to the Beagle Boys, "Of course you realize, this means . . . a skirmish."
Tarantulas mutters it in an episode of Beast Wars, after Blackarachnia turns on him (again).
During one of the "Spacecataz" segments, Oglethorpe and Emory have the following discussion after the Mooninites expose themselves at them:
Emory: Hey, maybe we should just back off. I mean, this is how fight starts.
Oglethorpe: This is not a fight... THIS IS A WAR!
Popeye in the face of abuse, he would finally roar, "I've had all that I can stands, and I can't stands no more!"
Non-humorous example on Gargoyles. When Xanatos turns Derek into a mutate, Elisa tells him that it's war now, and that she won't give up until she gets him back for it. It escalates to the point where when Fox turns into a werefox, Xanatos has to beg Elisa for help, and even then Goliath had to talk it over with her in private before she'd do it.
In The Smurfs episode "All's Smurfy That Ends Smurfy", the fairy Prince Garth declares war on the Smurfs for supposedly stealing their gold, which turns out to be the work of Gargamel.
The immediate lead-up to and course of World War I is famously complicated, but suffice it to say that this phrase got tossed around a whole lot right after Archduke Ferdinand was assassinated, as pretty much everyone wanted a war.
Austria-Hungary (Central Powers) wanted to use the assassination to destroy Serbia's independence and the threat it posed to her own internal stability.
Russia (Entente) wanted to expand her influence in The Balkans, was sure she could defeat Austria-Hungary, and was confident France would help if Germany got involved.
Germany (Central Powers) was deeply pessimistic about her chances in a war with France and Russia and hated to be dragged into a war over something as stupid as this, but was terrified of facing them alone.
France (Entente) was supremely confident that her army was the best in Europe, wanted to conquer Alsace-Lorraine, and cynically noted that now was the best time for a warnote Germany and Austria-Hungary had only nominally participated in the general European arms-race of 1911-14 - right now the Entente had a decisive material and trained-manpower advantage, but given time the Central Powers might find the money to counter that with spending of their own .
Britain (Entente) was looking for a convenient 'out' to dangerous levels of popular unrest at home and jumped at the chance to grab German colonies and German ships, and devastate Germany's economy.
Russia (Entente) wanted the end of the Ottoman Empire and chunks of Ottoman territory.
Japan (Entente) wanted German Qingdao, and German Pacific Territories.
Italy (Entente, 1915) wanted large chunks of Austro-Hungarian territory that the latter was not willing to give her.
Bulgaria (Central Powers, 1915) wanted parts of Serbia and Rumania.
Rumania (Entente, 1916) wanted parts of Hungary and Bulgaria.
The USA (Entente, 1917) wanted a say in the post-war peace settlement without sacrificing too many of her own people.
On October 28, 1940, the Italian Ambassador to Greece presented a three-hour ultimatum to Prime Minister Ioannis Metaxas, demanding that Italian troops be allowed free passage to unspecified strategic points throughout Greek territory. Today, Greeks acclaim Metaxas' reply via "Ohi Day," or "No! Day," but Metaxas' actual response was "Alors, c'est la guerre!"
The picture above is the USS Arizona burning in Pearl Harbor. Japanese Naval Marshal General Isoroku Yamamoto is alleged to have said:
President Roosevelt said something more true to the trope title: "Yesterday, December 7, 1941—a date which will live in infamy..." If this doesn't sound badass to you, remember what "infamy" means: what he essentially said was "This Is Unforgivable, and we shall never forget it." Less than one hour after those words were spoken, America officially declared war.
Charles Francis Adams was the American ambassador to England during the American Civil War. He was convinced that a British company was building warships for the Confederacy. He sent a series of notes to the British Foreign Minister insisting that the British government stop this, since England was supposed to be neutral. The last note contained the sentence: "It would be superfluous of me to point out to your Lordship that this is war." The British government bought the ships in question.
While not a formal declaration of war, when the government began executing the handful of captured rebel leaders of the Dublin Shoot-out/'Easter Rising' on the grounds that they'd crossed a line by killing innocent people, well-known (Irish) 'Home Rule' supporter John Dillon rushed to London and warned Asquith (Prime Minister of the time) to halt the executions, saying: "You are washing our whole life work in a sea of blood." This was promptly ignored, and over time public opinion in the city and the rest of west-Britain came down on the side of the killers who hadn't been in uniform.
Before the Six Day War, Israel specifically said that blocking the Straits of Tiran meant this. They weren't kidding.
Likewise several maritime powers have a standing "This Means War" on anyone who captures Istanbul. The reason for this is that Constantinople keeps the Russian fleet bottled up in the Black sea.
The Latin phrase for this is Casus Belli and this is the version often used in diplo-speak. Actually it sounds scarier in English then in Latin despite the reputation of the original Latin speakers.
After the attack on Saguntum in the beginning of the Second Punic War the Roman ambassador to Carthage went before the Carthaginian Senate and said that he carried in the folds of his toga peace or war and demanded that they choose. The Carthaginians said "you choose". So the Roman said he let fall war. And the Carthaginians said, "We accept it".