Mexican War, 1846–48, a war between the United States and Mexico. As a result of its victory the United States gained an area of 529,017 square miles (1,370,148 km 2 ) from Mexico—the present states of California, Nevada, and Utah, and parts of Arizona, New Mexico, Colorado, and Wyoming. In addition, the United States gained permanent possession of more than 275,000 square miles (712,000 km 2 ) of disputed territory claimed by Texas after its independence. The acquisition of these vast lands by the United States intensified the debate between North and South concerning slavery in the territories and brought the nation closer to civil war.

Zachary Taylor, one of the heroes of the Mexican War, was elected President in 1848. Two other leading participants, Winfield Scott and John C. Frémont, later ran unsuccessfully for the Presidency. Franklin Pierce (President, 1853–57) and Jefferson Davis (Confederate President, 1861–65) advanced their political careers by Mexican War service. Many of the Civil War generals, including Robert E. Lee and U.S. Grant, took part in the war, gaining valuable military experience.