The Mexican–American War (1846–1848): Causes, Course, and Consequences

The conflict that cost Mexico half its territory and made the U.S. a continental power.

Causes

• Texas Annexation (1845): Mexico never recognized Texan independence (1836) or U.S. annexation.
• Border Dispute: U.S. claimed Rio Grande; Mexico claimed Nueces River (150 mi north).
• Manifest Destiny: U.S. expansionist ideology—"sea to shining sea."
• Failed Diplomacy: Slidell mission (1845) to buy California/New Mexico rejected.

Course of War

Western Front (Kearny, Stockton, Frémont)

• June 1846: Kearny's Army of the West takes Santa Fe (NM) bloodlessly.
• July 1846: U.S. naval forces (Sloat, Stockton) seize Monterey, San Francisco, San Diego.
• Jan 1847: Last Mexican resistance in California ends (Treaty of Cahuenga).

Eastern Front (Taylor → Scott)

• May 1846: Taylor wins Palo Alto, Resaca de la Palma; crosses Rio Grande.
• Sept 1846: Taylor takes Monterrey after hard fight.
• Feb 1847: Buena Vista—Taylor defeats Santa Anna's larger army.
• Mar 1847: Scott lands at Veracruz (first major U.S. amphibious assault).
• Sept 1847: Scott enters Mexico City after Cerro Gordo, Contreras, Churubusco, Molino del Rey, Chapultepec.

Treaty of Guadalupe Hidalgo (Feb 2, 1848)

• Mexico cedes California, Nevada, Utah, Arizona, New Mexico + parts of Colorado, Wyoming (Mexican Cession, 525,000 sq mi).
• Rio Grande confirmed as Texas border.
• U.S. pays $15 million + assumes $3.25M in Mexican debts to U.S. citizens.

Impact

• U.S.: Continental nation, Gold Rush (1848), sectional crisis over slavery in new lands → Civil War.
• Mexico: Lost 55% of territory, political instability, lasting distrust of U.S.