American and European "Mountain Men" likely came through the area while exploring what is now central Arizona during the early 19th century. They obtained valuable beaver and otter pelts; these animals, as well as deer and Mexican wolves, often lived in the Salt River Valley when water supplies and temperatures allowed.
When the Mexican-American War ended in 1848, most of Mexico's northern zone passed to United States control and a portion of it was made the New Mexico Territory (including what is now Phoenix) shortly afterward. The Gadsden Purchase was completed in 1853. The land was contested ground during the American Civil War: both the Confederate Arizona Territory, organized by Southern sympathizers in 1861 with its capital in Tucson, and the United States Arizona Territory, formed by the United States Congress in 1863 with its capital at Fort Whipple (now Prescott) included the Salt River Valley within their borders. The valley was not militarily important, however, and did not witness conflict.
In 1863, the mining town of Wickenburg was the first to be established in what is now Maricopa County. At the time this county did not exist, as the land was within Yavapai County along with the other major town of Prescott.
The US Army created Fort McDowell on the Verde River in 1865 to quell Native American uprisings. Hispanic workers serving the fort established a camp on the south side of the Salt River by 1866 that was the first non-native settlement in the valley after the decline of the Hohokam. In later years, other nearby settlements would form and merge to become the city of Tempe, but this community was incorporated after Phoenix.