Before Professor X and Magneto were archenemies, they were closest of friends, working together with other mutants to stop the greatest threat the world has ever known.
X-Men: First Class
Before Charles Xavier and Erik Lensherr took the names Professor X and Magneto, they were closest of friends working together, with other Mutants to stop the greatest threat the world has ever known. In the process, a rift between them opened, which began the eternal war between Magneto's Brotherhood and Professor X's X-MEN.
