Brennus: In a world where superheroes and supervillains have been a common sight for almost a century, this is extremely common, and it seems there's almost an unwritten code-of-conduct for it: for a large enough threat, anyone is welcome, and will be allowed to walk away afterwards, even if their your Arch-Enemy. A pretty large number of story arcs are about the heroes and villains teaming up to tackle some larger threat like a Kaiju or the Spiteborn.
In Chrono Hustle Jack is running around the timeline, and Melinda has been assigned to capture him, but Jack doesn't want to be locked up, since they can't send him back to his own time. But then it turns out that there's some big conspiracy going on, and they end up working together.
In Cult of Personality, the oWn is trying to wipe out the RED and BLU corperations, and a band of mercenaries from both sides have to fight together to push oWn back.
In May Xnocens July and Borne are only enemies because of the questionable things July does to keep Idius March from being released. Once Idius March is released this becomes a moot point and they team up.
During a run on Bowser's Gnarly Party prior to Let's Play Every Mario Party, the other players realize that Steeler has all three bonus stars about halfway through the game and team up against him in minigames.
Reds! the threat of Nazi-Fascism forces the Comintern and capitalist powers to unite, on an individual it forces anti fascist conservatives like Winston Churchill to ally with International Communism against fascist barbarity.
They had talked of Roman history, the adventure of sailing on the high seas, and generally avoided politics. Meeting with class enemies had never felt so agreeable. "I have been relieved of any official responsibility to the workers' state. And while that does bring a certain aimlessness, I still try to help out from time to time. As you can imagine, it would not be politically possible for Premier Reed to meet with you or other British politicians directly. It would not be desirable for either of us. But as a private citizenů"
"Then let's cut to the chase," Churchill interrupted, "Hitler may well be the greatest threat to civilization since Attila the Hun. We can see that, even if the Tories cannot."
"I thought you considered yourself a Tory, Mister Churchill."
"Call me Winston. I owe you that much respect, from one soldier to another. Though we have our deep political disagreements, they are the kind that honest men have. It has been a bitter pill to swallow, but my visits to America have shown me that our two peoples are earnest in their belief in liberty. Yours is but a different conception than ours, befitting your own unique history. But I digress; I feel that the Conservatives no longer have anything to offer me or my country."
When Whateley Academy is attacked by a massive invading army, the usually bickering students drop everything to drive them off (though some of the Ultraviolents kept a few for the sake of... entertainment).
A key part of the Whateley Charter is that anyone, charter signatory or otherwise, who attacks any student's family as a way of getting to said student is to be treated as fair game by the signatories for anything up to and including orbital bombardment. Generally, those who have tried this in the past didn't survive long - this is one situation where the superheroes won't even try to restraint the supervillains.
Common practice in Worm, where both superheroes and rivaling super-villains will gang up on anyone causing enough trouble or breaking the unwritten rules, such as attacking a cape's family.
"Word gets around, and the cape community goes after the fucker. Protecting the status quo, keeping the game afloat. Bitter enemies call a truce, everyone bands together, favors get called in and everyone does their damndest to put the asshole down."
And when Endbringers attack, virtually every parahuman available will come together.
"This is why we are tolerated, why society allows and accounts for the capes that walk the streets and fight in its towns. Because we are needed for situations like this."
The Ruins of an American Party System: When the hung 1940 Presidential election heads into an equally divided House of Representatives, Republican candidate Alf Landon suddenly drops out of contention and throws his support behind Progressive incumbent Fiorello LaGuardia, in order to prevent Huey Long from potentially winning.
Subsequently, the 40s see the Progressives and Republicans forming an emergency coalition to guide the country through the wars against Japan and the South American fascists.
The incredibly complicated political setup of the 1946 midterms (brought about by fusion ballots and deep idealogical divides in all the parties), results in factions from the Republicans, Progressives, American Party, Socialists, and People's Coalition coming together to form a centrist grand coalition in the House of Representatives to oppose the more radical elements on both the left and right from possibly taking power.