One of the all-time crossover arcs in the Marvel Universe was Avengers vs X-Men, a massive event that has had lasting effects on the characters, teams and story arcs ever since. A great summary (with many spoilers) is available at Wikipedia.
Much of the story is available in trades Avengers vs X-Men on Amazon
…but I prefer reading on Marvel Unlimited. Here is a clickable list of links to the books that make up the story in order.