United States History: The Mexican-American War

Under the Treaty of Guadalupe Hidalgo, Mexico ceded over 525,000 square miles of territory to the United States, which amounted to about 55% of its pre-war territory.

The Mexican-American War officially ended with the signing of the Treaty of Guadalupe Hidalgo on February 2, 1848, which established the Rio Grande as the U.S.-Mexico border.

Following the fierce Battle of Buena Vista in February 1847, General Santa Anna's forces suffered heavy casualties, prompting him to withdraw his troops overnight and retreat toward Mexico City.

Santa Anna was in exile in Cuba when the war began. He made a secret deal with U.S. President Polk, promising that if the U.S. permitted him to bypass their naval blockade and return to Mexico, he would work to end the war on terms favorable to the U.S.

The United States did officially declare war on Mexico. President James K. Polk requested a declaration of war, which Congress approved on May 13, 1846.

While Mexico's president and congress declared a "defensive war" and recognized a state of war existed after U.S. troops invaded, Mexico never issued a formal, offensive declaration of war against the United States.

President James K. Polk was a strong proponent of Manifest Destiny, the 19th-century belief that the United States was divinely ordained to expand its territory across the North American continent to the Pacific Ocean.

The Mexican-American War (1846-1848) marked the first time in U.S. history that American military forces fought a major conflict primarily on foreign soil, invading and occupying parts of Mexico.