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The Mexican-American War. The Mexican-American War (1846-1848) redrew the map of North America. Its roots lay in Texas, which had broken from Mexico in 1836 and was annexed by the United States in 1845. Mexico had never accepted Texan independence, and the two nations disputed the border: Mexico claimed the Nueces River, the United States the Rio Grande. When American troops under Zachary Taylor marched into the disputed land in 1846 and blood was shed, President James K. Polk, an ardent expansionist, secured a declaration of war.
The war was fought on three fronts. Taylor's army won battles at Monterrey and Buena Vista in northern Mexico; an American column under Stephen Kearny seized New Mexico and California, where settlers had already proclaimed the Bear Flag Republic; and in the decisive campaign, General Winfield Scott landed at Veracruz in the first great amphibious operation of the American army and marched on Mexico City, storming the fortress of Chapultepec, where the young cadets remembered as the Ninos Heroes died in its defense. Mexico City fell in September 1847.
The Treaty of Guadalupe Hidalgo, signed in February 1848, ended the war on terms that transformed the continent. Mexico recognized the loss of Texas and ceded to the United States the vast territory of California, Nevada, Utah, Arizona, and parts of Colorado, Wyoming, and New Mexico, half of its national territory, in return for fifteen million dollars. Some hundred thousand Mexicans suddenly found themselves citizens of the United States.
The war made the United States a continental power but deepened the quarrel over slavery that would soon tear it apart: the question of slavery in the new lands led within a dozen years to civil war. In Mexico the defeat left a lasting wound and a long memory, and the war remains one of the most consequential, and most contested, in the history of both nations.
Library of Congress: Mexican War: A Resource Guide
Texas State Historical Association: Mexican War
PBS American Experience: The Mexican-American War
National Park Service: U.S.-Mexican War