Mexican-American War, also called Mexican War, Spanish Guerra de 1847, or Guerra de Estados Unidos a Mexico (“War of the United States Against Mexico”), war between the United States and Mexico (April 1846–February 1848) stemming from the United States’ annexation of Texas in 1845 and from a dispute over whether Texas ended at the Nueces River (Mexican claim) or the Rio Grande (U.S. claim). The war—in which U.S. forces were consistently victorious—resulted in the United States’ acquisition of more than 500,000 square miles (1,300,000 square km) of Mexican territory extending westward from the Rio Grande to the Pacific Ocean.
Mexico severed relations with the United States in March 1845, shortly after the U.S. annexation of Texas. In September President James K. Polk sent John Slidell on a secret mission to Mexico City to negotiate the disputed Texas border, settle U.S. claims against Mexico, and purchase New Mexico and California for up to $30,000,000. Mexican President José Joaquín Herrera, aware in advance of Slidell’s intention of dismembering his country, refused to receive him. When Polk learned of the snub, he ordered troops under General Zachary Taylor to occupy the disputed area between the Nueces and the Rio Grande (January 1846).
On May 9, 1846, Polk began to prepare a war message to Congress, justifying hostilities on the grounds of Mexican refusal to pay American claims and its refusal to negotiate with Slidell. That evening he received word that Mexican troops had crossed the Rio Grande on April 25 and attacked Taylor’s troops, killing or injuring 16 of them. In his quickly revised war message—delivered to Congress on May 11—Polk claimed that Mexico had “invaded our territory and shed American blood on American soil.”
Congress overwhelmingly approved a declaration of war on May 13, but the United States entered the war divided. Democrats, especially those in the Southwest, strongly favoured the conflict. Most Whigs viewed Polk’s motives as conscienceless land grabbing. And abolitionists saw the war as an attempt by the slave states to extend slavery and enhance their power when additional slave states were created out of the soon-to-be-acquired Mexican lands.
Following its original plan for the war, the United States sent its army on the Rio Grande, under Taylor, to invade the heart of Mexico while a second force, under Colonel Stephen Kearny, was to occupy New Mexico and California. Kearny’s campaign into New Mexico and California encountered little resistance, and the residents of both provinces appeared to accept American occupation with a minimum of resentment. Meanwhile, Taylor’s army fought several battles south of the Rio Grande, captured the important city of Monterrey, and defeated a major Mexican force at the Battle of Buena Vista in February 1847. But Taylor showed no enthusiasm for a major invasion of Mexico, and on several occasions he failed to pursue the Mexicans vigorously after defeating them. In disgust, Polk revised his war strategy. He ordered General Winfield Scott to take an army by sea to Veracruz, capture that key seaport, and march inland to Mexico City. Scott took Veracruz in March after a siege of three weeks and began the march to Mexico City. Despite some Mexican resistance, Scott’s campaign was marked by an unbroken series of victories, and he entered Mexico City on September 14, 1847. The fall of the Mexican capital ended the military phase of the conflict.
Polk had assigned Nicholas Trist, chief clerk in the State Department, to accompany Scott’s forces and to negotiate a peace treaty. But after a long delay in the formation of a new Mexican government capable of negotiations, Polk grew impatient and recalled Trist. Trist, however, disobeyed his instructions and on February 2, 1848, signed the Treaty of Guadalupe Hidalgo. According to the treaty, which was subsequently ratified by both national congresses, Mexico ceded to the United States nearly all the territory now included in the states of New Mexico, Utah, Nevada, Arizona, California, Texas, and western Colorado for $15,000,000 and U.S. assumption of its citizens’ claims against Mexico. (See the primary source document: Treaty with Mexico.)
Zachary Taylor emerged a national hero and succeeded Polk as president in 1849. The war reopened the slavery-extension issue, which had been largely dormant since the Missouri Compromise. On August 8, 1846, Representative David Wilmot of Pennsylvania attempted to add an amendment to a treaty appropriations bill. The Wilmot Proviso—banning slavery from any territory acquired from Mexico—was never passed, but it led to acrimonious debate and contributed greatly to the rising sectional antagonism. The status of slavery in the newly acquired lands was eventually settled by the Compromise of 1850, but only after the nation had come perilously close to civil war. In Mexico the war discredited the conservatives but left a stunned and despondent country. It also reinforced the worst stereotypes each country held about the other. Normalization of relations after the war proceeded slowly.