Last War Between the United States and Mexico: The Mexican–American War (1846–1848)

The United States and Mexico have fought one declared international war: the Mexican–American War (1846–1848). No full-scale war has occurred since.

Causes

• Texas Annexation (1845): U.S. admitted Republic of Texas; Mexico never recognized Texan independence, claimed border at Nueces River.
• Border Dispute: U.S. claimed Rio Grande; Mexico claimed Nueces River (150 mi north).
• Manifest Destiny: Polk administration sought California & New Mexico.

Course

• April 1846: Mexican cavalry attacked U.S. patrol in disputed zone (Thornton Affair). Polk: "American blood on American soil." Congress declared war.
• Northern Campaign (Taylor): Palo Alto, Resaca de la Palma, Monterrey, Buena Vista.
• California Campaign (Kearny, Stockton, Frémont): Bear Flag Revolt, quick conquest.
• Central Campaign (Scott): Amphibious landing at Veracruz, march to Mexico City — Cerro Gordo, Chapultepec, fall of capital (Sept 1847).

Treaty of Guadalupe Hidalgo (Feb 2, 1848)

Mexico ceded 55% of its territory — present-day California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado, Wyoming, Kansas, Oklahoma — for $15M + $3.25M assumed claims. Rio Grande confirmed as Texas border.

Subsequent Conflicts (Not "Wars")

• Border skirmishes / Bandit raids (1850s–1910s) — Cortina War, Garza Revolution.
• Pancho Villa Expedition (1916–1917): Pershing's Punitive Expedition into Mexico after Columbus, NM raid. Not a declared war.
• WWII: Mexico joined Allies (1942) — fought alongside U.S.
• Drug War / Security Cooperation (2000s–present): Merida Initiative, border militarization — not interstate war.

The 1846–1848 war defined the modern U.S. Southwest and left a legacy of contested memory, land grants, and bilateral tension that echoes in today's border politics.