Las Vegas is home to the largest Luxury Arcology on Earth, the The Jewel of the Mojave.
Outside the Arcology walls, American Motorized Combat League (AMCL) serves as a combination of military, mercenary army, raider group and combat sporting league. It successfully defended Las Vegas' borders from the incursions of La Sindicación ("La Sín") during the Second Mexican-American War ("La Reconquista"), at which point occurred the LasVegasSecession. And it's just as well, because that helps me avoid the absolutely disgusting yet somehow tempting pun "Sin(dicación) City."
The territory is officially claimed by United Mexican States, but neither side wants to engage in a costly and destructive war, so they are for now content to let the dispute rest.