The Mexican-American War

The United States, under President James K. Polk, against Mexico. Mexican civilians and the Native peoples of the borderlands were caught in the fighting.

Texas won independence from Mexico in 1836 but was not annexed for nearly a decade, partly because Mexico threatened war and partly because of Northern opposition to adding a slave state. In 1844 President John Tyler negotiated an annexation treaty, which the Senate rejected; with President-elect Polk’s support, a joint resolution of Congress passed in March 1845, and Texas entered the Union in December. Mexico broke diplomatic relations. The two countries also disagreed about Texas’s border: Texans claimed the Rio Grande, Mexico the Nueces River farther north.

Provocation and declaration (1845–1846). Polk, elected on an expansionist platform, ordered General Zachary Taylor into the disputed land between the Nueces and the Rio Grande, and sent John Slidell to Mexico to buy that strip along with New Mexico and California. When Mexico refused and skirmishes broke out in the disputed territory in the spring of 1846, Polk asked Congress for war; it declared war on May 13.

Northern Mexico and the West (1846–1847). U.S. forces campaigned in northern Mexico and took control of New Mexico and California.

The road to Mexico City (1847). U.S. forces under General Winfield Scott landed at Veracruz and fought their way inland, capturing Mexico City in September 1847.

Peace (1847–1848). State Department clerk Nicholas Trist, recalled by Polk, ignored the order and negotiated a treaty with Mexico’s commissioners, signed at Guadalupe Hidalgo on February 2, 1848.

• The occupation of the disputed border zone (1846): made armed clashes almost certain.
• Scott’s Veracruz landing and march inland (1847): brought the war to the Mexican capital.
• The fall of Mexico City (September 1847): led the Mexican government to negotiate.

Mexico ceded about 55 percent of its territory and recognized the Rio Grande as the border of Texas. The United States paid a $15 million lump sum and assumed debts the Mexican government owed to American citizens. The Senate ratified the treaty on March 10, 1848, after striking Article X, which had guaranteed the protection of Mexican land grants.

Together with Texas and the Oregon Treaty of 1846, the war brought under U.S. control the future states of the Southwest and the Pacific coast. Mexican citizens living in the ceded lands became residents of the United States, and the Senate’s removal of Article X, which would have guaranteed their land grants, left their property vulnerable. In January 1848, just before the treaty was signed, gold was discovered at Coloma, California, setting off the gold rush.

At home, the war was bitterly controversial. On August 8, 1846, Congressman David Wilmot proposed that “neither slavery nor involuntary servitude shall ever exist” in any territory taken from Mexico. Southern senators blocked the Wilmot Proviso, but it provoked a political firestorm that divided Congress along sectional lines and led to the Compromise of 1850 — and, a decade later, toward civil war.