In the early 1880s the people living in Mexico decided that they did not want to be ruled by Spain anymore. In 1810 Mexico went to war with Spain. In 1821 Mexico won the war and gained its freedom from Spain. Spain no longer had any control over Arizona and so the Spanish missionaries left, leaving the land back to the Indians.

Mountain men had been traveling westward kill animals for their fur. They traded these furs in other parts of the United States and told great stories about the Southwest. The United States became very interested in the land in the Southwest. Arguments between the United States and Mexico over land in the Southwest led to the Mexican War in 1846. The United States won this war in 1848 when the United States captured Mexico's capital. The land that was gained contained all of California, Utah, Nevada, a piece of Colorado, a piece of New Mexico and most of Arizona. In 1853 Mexico sold the land south of the Gila River to the United States in a purchase known as the Gadsen Purchase.

The land that is now Arizona joined the United States as part of New Mexico territory. It was owned by the U.S. but it was not a state. This land was important to the US because they wanted to build a railroad to California. Gold had been discovered in California in 1848 and they needed a railroad as a route to get there.

The Civil War began in 1861. The Southerners were fighting to keep slavery and the Northerners were fighting to abolish slavery. The Southerners wanted the Southwest territory so they could get to the gold in California. The Civil War ended in 1865. The Southerners lost the Civil War and slavery was abolished.

During the years of the Civil War, the people of Arizona decided that they no longer wanted to be part of the New Mexico Territory. Then, people began to find silver in Arizona and which made a stronger argument for Arizona to become its own territory. Miners and businessman fought hard to get Arizona to become a territory. On February 24, 1863 President Abraham Lincoln signed a law creating Arizona Territory.

Arizona was a territory for almost 50 years before it became a state.