The Mexican-American War (1846-1848)
The war between the United States and Mexico that redrew the map of North America. Fueled by the U.S. annexation of Texas, westward expansionism, and a disputed border, it ended with Mexico ceding more than half its territory — the present-day U.S. Southwest — and ignited the slavery controversy that led to the American Civil War.
Events
Texas Independence Leaves an Unresolved Border
Texas won independence from Mexico after the San Jacinto campaign, but Mexico refused to recognize the new republic. The two sides disputed whether the southern border of Texas lay on the Nueces River or, as Texas claimed, the Rio Grande — a gap of some 150 miles (240 km) of territory.
Location: Texas
The United States Annexes Texas
The U.S. Congress admitted Texas as a state. Mexico broke off diplomatic relations, viewing the annexation of what it still considered a rebellious province as an act of aggression. President James K. Polk had campaigned on expanding U.S. territory to the Pacific.
Location: Washington, D.C.
Blood on Disputed Ground and a Declaration of War
After a clash between Mexican cavalry and a U.S. patrol in the disputed strip on April 25, Polk told Congress that Mexico had "shed American blood upon American soil." Congress declared war on May 13 — but the claim was disputed at home: Abraham Lincoln introduced his "spot resolutions" demanding the exact location of the clash, and many Whigs called the conflict "Mr. Polk's War." Henry David Thoreau's act of tax refusal in protest, later published as "Civil Disobedience," dates from this period.
Location: Rio Grande / Washington, D.C.
Conquest of New Mexico and the Bear Flag Revolt
U.S. forces under Stephen Kearny occupied Santa Fe without a battle, while American settlers in Alta California declared the short-lived Bear Flag Republic at Sonoma. John C. Fremont's forces and the Pacific Squadron under Robert Stockton brought California under U.S. control by early 1847, though revolts in Los Angeles required re-capture.
Location: Santa Fe and Sonoma, Alta California
Monterrey and Buena Vista — the Northern Campaign
Zachary Taylor's army stormed the fortified city of Monterrey in September 1846. In February 1847, Taylor's smaller force defeated Santa Anna — who had returned from exile to lead Mexico's armies — at Buena Vista, breaking Mexico's last major offensive in the north.
Location: Monterrey and Buena Vista, Mexico
The Amphibious Landing at Veracruz
Winfield Scott landed the first large-scale U.S. amphibious operation at Veracruz, besieged the city, and took it after heavy bombardment. The city's civilian population suffered under the shelling. Scott then began the march inland toward Mexico City along the national road, supplied only from the coast.
Location: Veracruz, Mexico
Cerro Gordo and the Road to the Capital
Scott's army outflanked Santa Anna's fortified position at the Cerro Gordo pass, routing the Mexican army. Engineers including Robert E. Lee found the mountain trail that enabled the envelopment. The defeats at Cerro Gordo and subsequent actions caused Santa Anna's government to falter, but Mexico's leadership remained divided between those who would negotiate and those committed to continued resistance.
Location: Cerro Gordo, Mexico
The Fall of Mexico City and Chapultepec
After victories at Contreras, Churubusco, and Molino del Rey, U.S. forces stormed the Chapultepec fortress guarding the capital. The deaths of the young Mexican military cadets — the Ninos Heroes — became an enduring national symbol. Scott's army occupied Mexico City on September 14, and Santa Anna's government collapsed.
Location: Mexico City
Gold in the Ceded Lands and the Slavery Question
Days before the treaty was signed, gold was discovered at Sutter's Mill in the newly ceded California, triggering the 1849 gold rush and a surge of U.S. migration across the former Mexican territory. In U.S. politics, the question of whether slavery would expand into the new lands — posed by the Wilmot Proviso of 1846 and unresolved by the war — drove the country toward the Compromise of 1850 and, ultimately, the Civil War. In Mexico, the loss of roughly half the national territory became a central trauma in the country's historical memory, referred to in Mexican historiography as "the separation."
Location: Coloma, California
The Treaty of Guadalupe Hidalgo
The peace treaty ceded present-day California, Nevada, Utah, Arizona, and New Mexico, plus parts of other states — roughly half of Mexico's territory — to the United States for 15 million dollars, with the U.S. assuming some Mexican debts. Around 100,000 Mexican residents of the ceded lands were promised citizenship and property rights; in practice, many later lost land through litigation and violence. The U.S. Senate ratified the treaty over objections from both expansionists who wanted more and abolitionists who wanted less.
Location: Guadalupe Hidalgo, Mexico