Mexican-American War

The Mexican-American War, fought between 1846 and 1848, ended with the United States taking roughly half of Mexico's pre-war territory. The lands that passed to American control include all or most of present-day California, Nevada, Utah, and parts of Arizona, New Mexico, Colorado, and Wyoming. The war reshaped North America, intensified the American debate over slavery, and left a lasting mark on relations between the two countries.

The roots of the conflict lay in Texas. Mexico had won independence from Spain in 1821, and Anglo-American settlers moved into its northern province of Texas, many of them bringing enslaved people. Tensions over Mexican authority and the issue of slavery led to the Texas Revolution of 1835-36, and Texas became an independent republic. Mexico never recognized its loss. When the United States annexed Texas in 1845, the two nations quarreled over the southern border: Mexico pointed to the Nueces River, while Texas and the United States claimed the Rio Grande. President James K. Polk, an expansionist, also sought to purchase California and New Mexico, but Mexican leaders refused to negotiate.

Fighting began in the disputed territory in spring 1846, when clashes between American and Mexican forces led Polk to tell Congress that Mexico had shed American blood on American soil. Congress declared war. American armies under Zachary Taylor advanced from the Rio Grande into northern Mexico, while other forces seized New Mexico and California, where some American settlers had also staged the short-lived Bear Flag Revolt. The decisive campaign came in 1847, when Winfield Scott landed at Veracruz and marched inland to capture Mexico City. Mexico, weakened by political instability and poorly supplied, could not match American artillery and logistics, though its soldiers often fought bravely.

The Treaty of Guadalupe Hidalgo, signed in early 1848, set the Rio Grande as the Texas border and ceded the vast northern territories to the United States in return for a payment of 15 million dollars and the assumption of certain claims. Mexicans living in the ceded lands were promised the option of American citizenship and protection of their property, though in practice many faced discrimination and lost land. The Gadsden Purchase of 1853 later added a strip of southern Arizona and New Mexico.

The war was controversial in the United States from the start. Critics, including Abraham Lincoln, then a congressman, and Henry David Thoreau, questioned its justification, and many Northerners feared it would extend slavery. The question of whether slavery would spread into the new territories fed the crisis that led to the Civil War. In Mexico, the defeat became a lasting national trauma and a symbol of vulnerability to its powerful northern neighbor.