Legion War

The Legion War, also known as the NCR-Legion War, is an ongoing conflict in 2281, and is the catalyst for the main plot of Fallout: New Vegas.

Prelude
The two men most responsible for the founding of the Legion, Edward Sallow and Joshua Graham, met while Sallow, a Follower of the Apocalypse, was with a small party on a mission for the Followers, and Graham was doing missionary's work for the Mormon Church in New Canaan. Graham agreed to go with the Followers as his knowledge of the tribal languages of the East would greatly help with their mission.

The party was captured by the Blackfoot tribe, then at war with 'almost every other tribe in the Southwest'.