X-Men: First Class

Before mutants had revealed themselves to the world, and before Charles Xavier and Erik Lehnsherr took the names Professor X and Magneto, they were two young men discovering their powers for the first time.

Not archenemies, they were instead at first the closest of friends, working together with other Mutants (some familiar, some new), to prevent nuclear Armageddon. In the process, a grave rift between them opened, which began the eternal war between Magneto's Brotherhood and Professor X's X-Men.