Arizona states of Sonora and Baja California. Arizona is the 48th state and last of the contiguous states to be admitted to the Union, achieving statehood on February 14, 1912, coinciding with Valentine's Day. Historically part of the territory of "Alta California" in New Spain, it became part of independent Mexico in 1821. After being defeated in the Mexican–American War, Mexico ceded much of this territory to the United States in 1848. The southernmost portion of the state was acquired in 1853 through the Gadsden Purchase. Southern Arizona is known for its desert climate, with very hot summers and mild winters.
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