It was generally agreed by the American Mafia that Las Vegas was to be an open city where any of the families could operate. The American mafia families helped finance hotels and casinos by using loans from the Teamsters' pension fund. They installed front men to run the daily operations of the casinos.
In the 1950s, Michael Corleone was looking to move his empire to Vegas, having bought out several hotels. Moe Greene however, refused to pay up and was later assasinated by a Corleone button man. The family then moved over, with Fredo Corleone running the legitimate side of the hotels.
The Corleone's empire in Vegas was challenged by Hyman Roth in 1958. Roth acted as an ally to the Corleones, assisting them on buying out Meyer Klingman, but he secretly wanted their empire in ruins, and started a vicious chain of events that led to his death, and the death of Fredo Corleone.
By the 1960s, Michael Corleone had become the most powerful gambling czar in Las Vegas. However, in the late 1970s, Michael sold his casinos, in order to act as an honest businessman.