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 Admission to the Union The Admission to the Union Clause of the United States Constitution, often called the New States Clause, found at Article IV, Section 3, , authorizes the Congress to admit new states into the United States beyond the thirteen already in existence at the time the Constitution went into effect. The Constitution went into effect on June 21, 1788, after ratification by 9 of the 13 states, and the federal government began operations under it on March 4, 1789. Since then, 37 additional states have been admitted into the Union. Each new state has been admitted on