Regional definitions vary from source to source. New Mexico and Arizona (in dark red) are almost always considered the core, modern-day Southwest. The striped states may or may not be considered to be part of the same region. With the exception of Texas and Oklahoma (offset in blue) -- which are counted as part of the South -- the Southwestern states are also classified as West by the U.S. Census Bureau.
The Treaty of Guadalupe Hidalgo. The areas in the United States that were originally part of Mexico (white, dark brown). The Southwestern United States (also known as the American Southwest or simply the Southwest) is a region 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 around 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. The total population of these states is roughly 19 million people. Most of it was a part of the Viceroyalty of New Spain, in the Spanish Empire, during the Modern Era. California, Arizona, Nevada, Utah, and parts of New Mexico, Colorado, Wyoming, Oklahoma and Kansas were originally all part of Mexico before the Mexican-American War. Portions of some of the areas in the "divided" states, and including western parts of Texas, were those in dispute after the Texas Revolution.