After the Spanish founded Mission San Gabriel in 1771, the Chino Hills were used extensively for grazing by mission cattle. During the Mexican Republic era, the hills were used as spillover grazing from such surrounding Mexican ranchos as Santa Ana del Chino and La Sierra Yorba. The United States then conquered the territory in 1848 after battles with Mexican forces who declared US encroachment illegal. After this conquest, the land was still used primarily for grazing.