Formed by Edward Sallow (referred to as Caesar) and Joshua Graham in 2247, Caesars Legion was a major faction in the west of America and a major enemy of the New California Republic. Caesars Legion led several major campaigns again Tribal groups, east of the Colorado river which Caesar regarded as "savages". This ideology of civilizing the Tribal faction spawned from a research task overseen by the Followers of the Apocalypse in which 9 members were to research Tribal dialect in Arizona.
After campaigning in the East for sometime, Caesars Legion came at a crossroads with the NCR once a Legion base was established at Fortification Hill. Relations were hostile as both powers sought to conquer their respective side of the river which escalated into a bloody feud. A major asset, the Hoover Dam, was the key to pushing back the losing forces. What is now known as the First Battle of Hoover Dam eventually saw the Legions first decisive defeat. While this war remained stagnant, it was simply prolonging a second battle, with the help of the renown Courier, saw a victory for neither the NCR nor the Legion, but for an independent Vegas, headed by a powerful leader at the Lucky 38.
After this defeat, Legion forces retreated. Many survivors believe the fate of the Legion was at the mercy of the NCR once The Cloud was developed. As retaliation for a chemical attack at Camp Searchlight, the virus was released over Legion controlled portions of the Mojave and Colorado.
The Legion are simply another legend spread around by survivors just like the many tribes and communities destroyed by the Cloud. Remnants of the war can still be found, 20 years later.