In the aftermath of the Texan Revolution and the Mexican War, a large number of Mexicans suddenly found themselves in the United States. Borders were redrawn, and half of what used to be Mexico became part of the United States. The territories eventually became the states of Texas, California, New Mexico, Nevada; and parts of Colorado, Arizona and Utah.
Most of the Mexican residents of these areas remained, despite the realignment of power. Although most were given at least nominal rights, such as suffrage, many faced discrimination and a growing alienation from the land that had once been their home. As settlers from the United States moved out west, especially to California during the gold rush of the late 1840s and 1850s, Mexicans became a minority. This minority status, coupled with the often extreme prejudice to which they were subject, caused their power and involvement in policy- and decision-making to wane. Many Mexican Americans, especially in Texas, had their right to vote denied by intimidation and exclusionary tactics such as "white primaries." Even property rights were violated as the growing population of American settlers who simply took over the land of Mexican Americans by declaring squatters rights, or used the court system to obtain ownership Mexican land grants. These actions were in clear violation of the Treaty of Guadalupe Hidalgo, which had ended the Mexican War.
It is not clear what role Mexican Americans played in the Civil War. They were concentrated in the West, which saw few Civil War battles, and in which the recruiting system must have been hardly established relative to recruitment in the North, South, Old Northwest and Old Southwest. The American settlers in the former Mexican territories may have also felt reluctant to allow the equipping and training of people with whom they had recently fought a war of territorial aggression, and with whom they still experienced serious tensions.