What was the Mexican-American War?

The Mexican-American War was a conflict between the United States and Mexico fought from May 13, 1846 to February 2, 1848, ending in a decisive American victory that produced the Mexican Cession of about 525,000 square miles of territory and reshaped the geography of North America. The roots of the war lay in the annexation of Texas. Texas had won independence from Mexico in 1836 but Mexico refused to recognize Texan sovereignty. The United States annexed Texas on December 29, 1845 by joint resolution rather than treaty, and President James K. Polk pressed for further territorial expansion to the Pacific.

A boundary dispute over whether the southern border of Texas was the Rio Grande (American claim) or the Nueces River 100 miles to the north (Mexican claim) became the immediate cause. Polk sent General Zachary Taylor with about 4,000 troops to the disputed area in early 1846. Mexican forces crossed the Rio Grande on April 25, 1846 and skirmished with American patrols, killing or capturing about 60. Polk asked Congress on May 11, 1846 to recognize that war already existed, and Congress declared war on May 13, 1846 by 174 to 14 in the House and 40 to 2 in the Senate, with John Quincy Adams and a small group of Whigs and abolitionists opposing.

The war proceeded on three fronts. In northern Mexico, Zachary Taylor defeated Mexican forces at Palo Alto and Resaca de la Palma in May 1846, captured Monterrey in September 1846, and won the Battle of Buena Vista against General Antonio Lopez de Santa Anna in February 1847. In California and New Mexico, Stephen W. Kearny's Army of the West captured Santa Fe in August 1846 and joined naval forces in California. John C. Fremont led a smaller force and worked with the Bear Flag Republic, an American settler revolt at Sonoma in June 1846. American forces controlled California and New Mexico by early 1847.

The decisive campaign ran from Veracruz to Mexico City under General Winfield Scott. Scott's force of about 12,000 landed at Veracruz on March 9, 1847, captured the city, and marched 260 miles inland fighting battles at Cerro Gordo, Contreras, Churubusco, Molino del Rey, and Chapultepec. American forces took Mexico City on September 14, 1847. Diplomat Nicholas Trist negotiated the Treaty of Guadalupe Hidalgo signed February 2, 1848 in which Mexico ceded California, Nevada, Utah, most of Arizona and New Mexico, and parts of Colorado and Wyoming for 15 million dollars and assumption of 3.25 million dollars in claims. About 13,000 Americans died in the war, mostly from disease.