The New California Republic or NCR as most folks called it. The NCR began its life in the small farming community of Shady Sands. It wasn't until the year 2186 that the republic was officially founded. Under the rule of its second president the republic opened trade routes with the Hub and Junktown. The republic persuaded the two towns to join them, the NCR kept on growing. The majority of South California was under NCR rule, the republic kept on pushing east until it came to the Mojave Wasteland.
The NCR pushed into the Mojave, they made it all the way to Hoover Dam and realized the military potential of holding it, and the NCR sent a detachment to the city of Las Vegas which had been renamed New Vegas. When they arrived they found an army of Securitrons, one of the robots approached the NCR and wished parlay, and they found out that the "owner" of New Vegas was Robert House.
In the end the New Vegas treaty was negotiated, the NCR was given Hoover Dam and was allowed to divert 95% of the power to California but the other 5% had to be diverted to New Vegas.
So the NCR had controlled the Mojave (expect New Vegas), but it wasn't their fate to have a peaceful time. From the east another group forged from the conquest of 86 tribes, Caesar's Legion entered the Mojave. The Legion attacked the dam; the NCR was barely able to defeat them. But the Legion wasn't destroyed oh no. It came back and attacked the Dam again; the Legion would've won if it wasn't for the timely arrival of the courier John Edwards.
The Legion scattered and NCR reigned supreme.