Sometimes, Marvel crossovers involve various superheroes teaming up to take down some larger threat, but just as often, they revolve around those same superheroes going head-to-head over some mix-up or another. It really doesn’t take much to pit two vigilantes - or two superheroic teams - against one another, and then it’s time for a big, flashy Avengers vs. X-Men-style showdown.
For all the times that heroes like Captain America and Iron Man have united to save the world, they’ve also caused some serious devastation on Earth-616 by going to straight-up war with one another. Occasionally, this takes the form of one set of supes or another being manipulated into working with the villains. More often, however, it's just a moral disagreement or a big misunderstanding - with the consequences just as dire either way.
There isn't always a winner when two teams of superheroes battle it out, but there are always losers.