Mexican War (1846–48), conflict between Mexico and the United States that resulted in the defeat of Mexico and America's acquisition of territory that became California, Nevada, Utah, most of New Mexico and Arizona, and parts of Colorado and Wyoming. The war took place against a background of expansionist sentiment (Manifest Destiny) in the United States, which held that it was destined to become a continental power and the dominant nation of the Western Hemisphere.
Causes of the war
In 1835, when the region comprising Texas, then under control of Mexico, revolted and declared its independence, Mexico warned the United States that it would break off diplomatic relations if Texas were admitted to the union. President James K. Polk, elected in 1844, favored expansionism and backed the annexation of Texas. When Texas was accepted into the union in 1845, Mexico broke relations. The matter could have been negotiated peacefully except that other issues, notably the boundary dispute between Texas and Mexico, stymied agreement. Mexico put the boundary at the Nueces River while Texas claimed it was the Rio Grande farther to the south. In addition, American citizens claimed damages from the Mexican government for losses sustained in Mexico's war of independence from Spain, which ended in 1821. Also, U.S. expansionists had designs on the vast California territory, then under Mexican rule but which had experienced an influx of English-speaking people. Polk sent John Slidell to offer Mexico a solution whereby the United States would pay Mexico $25 million and assume all American claims against it if Mexico, in turn, would accept the Rio Grande boundary and agree to sell the New Mexico and California regions to the United States. When Mexico declined to deal on these terms, Polk resorted to force.
Outbreak of the war
In April 1846, General Zachary Taylor was ordered to advance his 3,000 troops from the Neuces to the Rio Grande. Mexico saw this as an invasion, and a slight engagement of forces occurred—excuse enough for Polk, on May 13th, to get a declaration of war on Mexico. Even before that reached Taylor, he had won the battles of Palo Alto and Resaca de la Palma, forcing the Mexicans across the Rio Grande.
Course of the war
U.S. strategy involved a 3-pronged attack on Mexico: an invasion of New Mexico and California, a naval blockade of both the Gulf of Mexico and California, and a major thrust from the north. The first 2 objectives were quickly attained. General Stephen W. Kearny, with about 1,700 troops, took New Mexico in August 1846, then moved on to California where, in January 1847, forces under Kearny and Commodore Robert F. Stockton won the Battle of San Gabriel, completing the conquest of California. Taylor took Monterrey in September 1846, but Mexico still refused to negotiate. Polk sent General Winfield Scott to land an army at Veracruz on Mexico's east coast, and to menace Mexico City. Many of Taylor's best troops were transferred to the Veracruz campaign, leaving him vulnerable when challenged by a large Mexican force led by General Antonio Lopez de Santa Ana. In February 1847, Taylor only narrowly won the hard-fought battle of Buena Vista. Scott took Veracruz in March and drove toward Mexico City. In April, his troops defeated Santa Ana at the mountain stronghold of Cerro Gordo; and in August, near Mexico City, after hard fighting they defeated Mexican forces at the battles of Contereras and Churubusco. A 2-week armistice ensued, but when negotiations broke down, fighting resumed. Moving on Mexico City, Scott took the strong points of Casa Mata and Molino del Rey and stormed the fortress of Chapultepec. On September 14, 1847, U.S. troops entered Mexico City, ending the war.
Outcome of the war
By the Treaty of Guadalupe Hidalgo (February 1848), Mexico ceded to the United State s two-fifths of Mexican territory (nearly all of present-day Arizona, California, Colorado, Nevada, New Mexico, and Utah)—more than 500,000 sq mi. The United States agreed to pay Mexico $15 million and to assume all outstanding claims of American citizens against Mexico. The war deeply divided the American people, not least because some feared an extension of slavery into the new territories. The Compromise of 1850 made California a free state but allowed the people of the other territories to decide whether they should be slave or free states. Bitter disputes followed, 12 years later contributing to the chain of events that led to the American Civil War.