Arizona Territory 1877. The capital was finally moved to Phoenix on February 4, 1889. The territory was admitted to the Union as the 48th state on February 14, 1912. Proclamation to the People of Arizona. Arizona Territory The Territory of Arizona (also known as Arizona Territory) was a territory of the United States that existed from February 24, 1863 until February 14, 1912, when the remaining extent of the territory was admitted to the Union as the state of Arizona. It was created from the western half of the New Mexico Territory during the American Civil War. Following the expansion of the
Admission to the Union phrase, "new States shall be admitted on the same terms with the original States", in the "new states" clause was defeated. That proposal would have taken the policy articulated in the Ordinance of 1784 and made it a constitutional imperative. Many delegates objected to including the phrase however, fearing that the political power of future new western states would ultimately overwhelm that of the established eastern states. Delegates, understanding that the number of states would inevitably increase, did agree to include wording into this clause to preclude formation of a new state out of an established one without the consent