X-Men: First Class

X-Men: First Class (2011) — 1962. Young Xavier (James McAvoy) and Erik (Michael Fassbender) meet, recruit the first team, and split over the Cuban Missile Crisis. This is the emotional origin of everything.

Sebastian Shaw — the Nazi scientist who murdered Erik's mother in front of him to force his power to manifest — is running the Cuban Missile Crisis from both sides, intending to trigger a nuclear war that leaves mutants inheriting the Earth. Charles and Erik recruit the first team to stop him, and do.

The split comes immediately afterwards. With Shaw dead, the American and Soviet fleets turn their guns on the mutants who just saved them. Erik takes that as proof; Charles takes it as a reason to try harder. In the crossfire Charles is shot in the spine, and Mystique — raised alongside him — leaves with Erik. Every later film is an argument between these two positions.