Caesar's Legion (commonly referred to as the Legion) was the autocratic, traditionalist recreation of the famous Roman Empire, wiped out after the Second Battle of Hoover Dam.
Formed by Edward Sallow (referred to as Caesar) and Joshua Graham in 2247, Caesar's Legion was a major faction on the West Coast, and a major enemy of the New California Republic. Caesar's Legion led several major campaigns against tribal groups east of the Colorado River, regarded by Caesar as savages. This ideology of uniting the tribes spawned from a research task overseen by the Followers of the Apocalypse, where 9 members, including Caesar and Graham, were to research tribal dialects in Arizona.
After campaigning east of the Mojave Desert for some time, Caesar's Legion came to 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, escalating into a bloody feud. A major asset, the Hoover Dam, was seen as key to pushing back the losing forces. What became known as the First Battle of Hoover Dam eventually saw the Legion's first decisive defeat. A period of prolonged tension between the two sides foreshadowed a second battle over the Dam. With the aid of the Courier, the NCR triumphed over the Legion and took permanent control of the site.
Leaderless and demoralized, 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 radioactive fog 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.