Last US-Mexico War: Mexican-American War (1846–1848)

The **last declared war** between the United States and Mexico was the **Mexican–American War**:

• Start: **April 25, 1846** (Thornton Affair — Mexican cavalry attacked US patrol in disputed territory north of Rio Grande).
• Declaration: **May 13, 1846** (US Congress).
• End: **February 2, 1848** — **Treaty of Guadalupe Hidalgo** signed.
• Ratification: US Senate (Mar 10, 1848), Mexican Congress (May 1848).

Outcome

Mexico ceded **~55% of its territory** (present CA, NV, UT, AZ, NM, CO, WY) — **Mexican Cession**. US paid **$15M** + assumed $3.25M Mexican debts to US citizens. Rio Grande confirmed as TX border.

Since Then: Conflicts, Not War

• Border Skirmishes / Incursions: **Pancho Villa Expedition (1916–17)** — Pershing pursued Villa into Mexico after Columbus, NM raid. **Not war.**
• Drug War / Cartel Violence (2006–present): US provides intelligence, training, equipment (Mérida Initiative). **Not war.**
• Immigration / Border Enforcement: Ongoing policy friction. **Not war.**

Key Distinction: No **declaration of war** or **authorization for use of military force (AUMF)** against Mexico since 1848. Relations are **diplomatic, economic, security cooperation** — deeply intertwined, sometimes tense, but **peaceful**.