The first bill for statehood was introduced and defeated in Congress in 1889. Congress wanted Arizona and New Mexico to be a single state. Arizonians voted in 1906 to reject this plan, and in January 1910 they held a constitutional convention within the state to begin writing their state constitution.

In 1912, Arizona became the 48 th state with Phoenix as the state capital. This completed the continental United States.

In 1917, the United States joined World War I against Germany. The Zimmerman Telegram was one reason we joined the war. It was sent from Germany to Mexico, and said that if Mexico helped Germany fight in the war, Mexico would regain Arizona.

In 1911, President Roosevelt dedicated a dam that was named after him. The Coolidge Dam, the Bartlett Dam, and the Hoover Dam followed.

During World War II, Army Air Corps pilots trained in Arizona. The resources of cotton, copper, and beef were needed to provide materials for the war. These new job opportunities, and the introduction of air conditioning attracted people from all over the country to Arizona.

Since the 1950s, Arizona's population has grown quickly. It almost tripled from 1960-1990. With this rapid growth came a need for more water. In 1963, the Supreme Court decided on a proper division of water from the Colorado River, between Arizona, Nevada and California. However, more water was needed. In 1985, the Central Arizona Project brought more water from the Colorado River by pipeline to Phoenix, and in 1991 to Tucson. The project includes 541 km (336 mi) of pipeline and cost $3.7 billion. 

In 1974 the US Congress divided the Hopi Reservation between the Hopi and the Navajo Indians.