Mexican-American War

The Mexican-American War was fought between the United States and Mexico from 1846 to 1848. Tensions rose after the U.S. annexed Texas in 1845, which Mexico still considered its territory, and the two countries disputed whether Texas's southern border lay at the Rio Grande, as the U.S. claimed, or the Nueces River, as Mexico claimed. President James K. Polk, an ardent expansionist, sent troops under Zachary Taylor into the disputed zone; after a clash with Mexican forces, Congress declared war in May 1846.

American armies won a string of victories, including campaigns in northern Mexico, the occupation of California and New Mexico, and Winfield Scott's capture of Mexico City in 1847. The war reflected the spirit of manifest destiny — the belief that the United States was destined to expand across the continent — though critics, including many Whigs and a young Abraham Lincoln, condemned it as an unjust land grab.

The Treaty of Guadalupe Hidalgo (1848) ended the war. Mexico ceded roughly half its territory — the Mexican Cession, including present-day California, Nevada, Utah, and parts of Arizona, New Mexico, Colorado, and Wyoming — in exchange for $15 million, and the U.S. border was fixed at the Rio Grande. The new lands immediately reignited the slavery question: the Wilmot Proviso's failed attempt to ban slavery in the acquired territory polarized Congress and set the stage for the Compromise of 1850.

The AP US History exam tests the war's causes (Texas annexation, the border dispute, manifest destiny), its outcome in the Treaty of Guadalupe Hidalgo, and above all its consequences — how the Mexican Cession deepened sectional conflict over slavery in the decade before the Civil War.