The United States acquired most of the area of Arizona from Mexico in 1848. The United States acquired additional area, comprising the southern part of Arizona, from Mexico in 1853 as part of the Gadsden Purchase. Arizona Territory was organized from the western part of New Mexico Territory on February 24, 1863. Part of Arizona Territory north and west of the Colorado River was added to the state of Nevada in 1866, resulting in generally the same boundary as the present state of Arizona. Arizona was admitted to the Union on February 14, 1912, as the 48th state.