Arizona north of the Gila River was annexed to the United States as a result of the Mexican War, 1846-1848. It became part of New Mexico Territory in 1850. The area south of the Gila River was included in the Gadsden Purchase. Southern Arizona was settled principally by Americans from the South. During the Civil War, there was considerable sentiment for the Confederacy. One military engagement occurred in Arizona, at Pichacho Pass, in April 1862.

Much settlement occurred following the Civil War, along with expanded silver and copper mining and cattle and sheep ranching. In 1878 the railroad reached Yuma from California. A state constitution covering what was New Mexico and Arizona was rejected. In 1910 Arizona was permitted to draft its own constitution, which was vetoed the following year by President William Howard Taft because it provided for the recall of judges by popular vote. With that provision deleted from a new constitution, Arizona was proclaimed the 48th state on February 14, 1912.