The Mexican War of 1846-1848 broke out ostensibly over a dispute about the Texan boundary. But it really originated in the expansionist spirit of “manifest destiny,” of which President James K. Polk was a leading proponent. Polk came to office determined to acquire the Mexican provinces of California and New Mexico and when Mexico refused to sell them, used a border clash as a pretext for war. Although American opinion was deeply divided over the conflict, the war turned out to be a succession of triumphs for American arms. The two provinces Polk coveted were easily overrun: General Zachary Taylor overwhelmed a much larger Mexican army at Buena Vista (February 1847); and General Winfield Scott’s seaborne expedition to Vera Cruz fought its way into the heart of Mexico against superior forces and captured Mexico City (September 1847), forcing Mexico to make peace. By the Treaty of Guadalupe-Hidalgo (February 1848), Mexico ceded the territories of California and New Mexico and acknowledged the Rio Grande as the Texas boundary. While the war brought the U.S. vast territorial gains — extending America’s western boundary to the Pacific — it also revived the simmering conflict over slavery and its extension to new territories.

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