U.S. Acquisition and the Discovery of Minerals

In the Treaty of Guadalupe Hidalgo (1848), ending the Mexican War (1846–48), Mexico relinquished control of the area N of the Gila River to the United States. This area became part of the U.S. Territory of New Mexico in 1850. The United States, wishing to build a railroad through the area S of the Gila River, bought the area between the river and the S boundary of Arizona from Mexico in the Gadsden Purchase (1853).

Arizona’s minerals, valued even by prehistoric miners, attracted most of the early explorers, and although the area remained a relatively obscure section of the Territory of New Mexico, mining continued sporadically. Small numbers of prospectors, crossing Arizona to join the California gold rush (1849), found gold, silver, and a neglected metal—copper.

In 1861, at the outbreak of the Civil War, conventions held at Tucson and Mesilla declared the area part of the Confederacy. In the only engagement fought in the Arizona area, a small group of Confederate pickets held off Union cavalry NW of Tucson in the skirmish known as the battle of Picacho Pass.

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