Mexican War

Mexican War (1846–48) a war that vastly increased U.S. territory and contributed to the sectional crisis. Elected in 1844 on a platform of expansionism, James K. Polk moved quickly to fulfill his promises, obtaining Oregon from Britain and negotiating to buy California and other Mexican territories. Mexican President Mariano Paredes predictably rejected American offers. Already seething from the U.S. annexation of Texas in 1845 and the attempts to stretch the border to the Rio Grande, he declared a state of “defensive” war on April 23, 1846. When word of a Texas skirmish eventually reached Washington, Polk asked Congress to acknowledge war, which it did on May 13.

Although Mexico's regular army owned superior numbers, the U.S. army possessed better training, leadership, and weaponry; and over 73,000 volunteers. The United States mounted several nearly simultaneous offensive actions. The fighting began on May 8, and Gen. Zachary Taylor's troops won the Battle of Palo Alto (near Brownsville, Texas). As they moved steadily southward, capturing Monterrey by late September and Saltillo in November, Col. Stephen Watts's troops easily occupied Santa Fe, from which they began an overland march to California. Aided by the navy, the army captured California by early January. Around the same time, Alexander Doniphan's forces occupied Chihuahua, and the United States won every battle in the first phase of the war.

While Polk and Winfield Scott drew up plans to capture Mexico City and thus force surrender, the new Mexican president, Gen. Antonio López de Santa Anna, rebuilt his army and prepared to attack Taylor's forces at Saltillo. Helped by an intercepted letter containing Santa Anna's plans and by Col. Jefferson Davis's Mississippi volunteers, Taylor withstood the attack. As Santa Anna retreated to the capital, Scott captured Veracruz, from where the march to Mexico City began. Scott's troops eventually reached the outskirts of the city by mid August, and, after a series of battles, U.S. forces entered Mexico City on September 14. The Treaty of Guadalupe-Hidalgo was signed on February 2, and confirmed by both sides on May 30, 1848, officially ended the war.

The United States gained the territories that would become California, New Mexico, Arizona, Nevada, Colorado, and Utah, and stretched the Texas border to the Rio Grande. On a federal level, Polk had greatly expanded the powers of the commander in chief. On the strength of his war record, Taylor won the 1848 election. Scott, in turn, was the Whigs' presidential nominee in 1852, but lost to his former subordinate, Democrat Franklin Pierce. Most importantly, the war heightened tensions over slavery, as a series of postwar measures granted slavery in California, abolished it in Washington, D.C., and created the new Fugitive Slave Law, thus bringing the country closer to the Civil War.