Mexican-American War

The Mexican-American War (1846–1848), known in Mexico as the Intervención estadounidense en México, was triggered by disputes over the U.S. annexation of Texas in 1845 and conflicting claims to the territory between the Nueces River and the Rio Grande. After a skirmish between U.S. and Mexican troops in the disputed zone in April 1846, President James K. Polk asked Congress for a declaration of war, which was approved on 13 May 1846.

U.S. forces under Zachary Taylor advanced into northern Mexico, winning at Palo Alto, Resaca de la Palma, Monterrey, and Buena Vista. A second front under Stephen W. Kearny seized New Mexico and, with naval support, California. The decisive campaign was Winfield Scott's amphibious landing at Veracruz in March 1847 and his march inland, culminating in the capture of Mexico City in September 1847 after battles at Cerro Gordo, Churubusco, Molino del Rey, and Chapultepec — the last famous for the Niños Héroes.

The war ended with the Treaty of Guadalupe Hidalgo, signed on 2 February 1848. Mexico ceded roughly half its national territory — present-day California, Nevada, Utah, and parts of Arizona, New Mexico, Colorado, and Wyoming — in exchange for US$15 million and assumption of certain claims. The Rio Grande was confirmed as the Texas boundary. The 1853 Gadsden Purchase later adjusted the southern border.

Politically, the war intensified the U.S. debate over the expansion of slavery into new territories, foreshadowed by the Wilmot Proviso (1846), and is widely cited as a precursor to the American Civil War. In Mexico, the loss destabilized successive governments and reshaped national identity. The conflict is frequently invoked in IR scholarship as a textbook case of expansionist war, Manifest Destiny, and asymmetric outcomes producing lasting territorial grievances.