The Southwestern United States (also known as the American Southwest or simply the Southwest) is a region of the United States defined in different ways by different sources. Broad definitions include nearly a quarter of the United States, including Arizona, California, Colorado, Nevada, New Mexico, Oklahoma, Texas, and Utah. Narrowly defined, the "core" Southwest is centered on the Four Corner states, with parts of the other states making up the beginnings and endings of the Southwest. The five main southwestern states, Arizona, New Mexico, Colorado, Utah, and Nevada, are also all considered part of the Mountain West as well as the Southwest. Using the broad definition (and not counting California cities), the total population of these states is roughly 19 million people; and the top five most populous cities in the Southwest (in order) are: Phoenix, El Paso, Denver, Las Vegas, and Albuquerque.
Most of it was a part of the Viceroyalty of New Spain, in the Spanish Empire. Prior to Skydark, what is now California, Arizona, Nevada, Utah, New Mexico, and portions of Colorado, Wyoming, Oklahoma and Kansas were part of Mexico, before the Mexican-American War and the Gadsden Purchase of 1853. Portions of some of the areas in the "divided" states, and including western parts of Texas, beginning in San Antonio, were those in dispute after the Texas Revolution.
The Southwest, even with the massive destruction wrought by the final war, area's such as Nevada, Arizona and New Mexico survived relatively unchanged as they have for thousands of years. The other areas such as Texas, were heavily affected. Originally, they were hot and dry. Quite often new rivers were being created as the storms changed the terrain. Acid storms and boiling winds would blow in from the Gulf, killing anyone unprotected. The swamps and bayous of Louisiana became toxic cesspools of mutant gators and humanoids. Many of the cities in this area were hit by neutron weapons, killing the people, but leaving the cities relatively intact.