Tensions mount between the expanding United States and Mexico during the 1830s and come to a boil in 1836, when Americans living in Texas, then part of Mexico, rebel against Mexican authority. Antonio López de Santa Anna leads a Mexican army against the insurgents but fails to stop the rebellion. Nine years later, the U.S. annexes Texas and the two countries become embroiled in a border dispute.
President Polk orders the Army of the West to take over New Mexico, while John C. Fremont leads a rebellion that overthrows Mexican authority in California. Fremont's followers initially declare themselves an independent republic, but three weeks later decide to join the United States. Mexican settlers in both territories fight in vain to reclaim their land, and U.S. forces move south toward Mexico City.
Santa Anna returns from exile in Cuba and is again elected president of Mexico. American President Polk orders a two-pronged attack on Mexican territory: Zachary Taylor's army move in overland from the north, while Winfield Scott leads an attack on Veracruz by sea. After defeating Santa Anna at Cerro Gordo and capturing Veracruz, the U.S. troops continue their push toward the capital.
American troops capture Mexico City after heavy fighting, and the war ends. The Treaty of Guadalupe Hidalgo cedes Upper California and all the states of the present-day American Southwest to the United States.