Introduction to the Mexican-American War (1846–1848)

Though often overlooked by historians, the Mexican-American War of 1846–1848 presaged dramatic change in America. The conflict emerged out of America's expansionist past and sped the nation on its way to the Civil War.

In the early nineteenth century, thousands of Protestant Anglo-Saxon Americans had poured into the Mexican territory of Texas. These settlers, led by Stephen Austin and Sam Houston declared their independence in 1835. Mexican President Antonio de Santa Anna personally led a punitive expedition to quash the rebellion. Initially, his army fared well, overcoming fierce resistance at the Alamo in San Antonio. But the Mexican victory provoked furious—and sustained—hatred from the American Texans. A month later, Santa Anna was defeated and captured at the Battle of San Jacinto. Santa Anna was released only after he had signed a treaty acknowledging Texas as a sovereign state.

Santa Anna returned home in disgrace. Infuriated, the Mexican government repudiated the treaty and refused to recognize the Republic of Texas. For the next decade, Mexican and Texan troops fought a sporadic border war, with raids, incursions, and atrocities on both sides.

In March of 1845, Texans agreed to annexation by the United States. Mexico immediately broke off diplomatic relations with Washington.

Expecting a Mexican invasion of Texas, U.S. President James K. Polk preemptively sent General Zachary Taylor to the Rio Grande River with four thousand men. Across the river, General Pedro de Ampudia's forces saw Taylor's army as an invasion into their territory. The Mexican government declared war on April 23, 1846. Congress responded in kind on May 13. Fighting had already begun.

Taylor moved west into the heart of Mexico, winning several bloody victories. On February 24, 1847, at Buena Vista, his army drove back a vastly larger Mexican force commanded by Santa Anna himself.

In the meantime, Colonel Stephen Kearny captured Santa Fe, New Mexico, without resistance. His army then continued to California, where the Pacific Squadron, under Commodore John Sloat, had taken San Francisco and other key ports on the Pacific Coast.

Colonel Alexander Doniphan had traveled with Kearny to Santa Fe and then turned south into Mexico. His forces defeated the Mexicans at El Brazito on Christmas Day, 1846. Two days later they occupied El Paso, and then they marched south towards Monterrey.

General Winfield Scott, with the Navy's Home Squadron, fought from Vera Cruz in March 1847 to Mexico City in September. American troops occupied the city until the peace was concluded. Santa Anna fled the country.

After two years of intense fighting, both armies had suffered enormous losses. The Americans, though almost always outnumbered, had won every battle, largely due to their superior weaponry. By the end of 1847, they controlled New Mexico, California, and a substantial part of Mexico.

On February 2, 1848, the Treaty of Guadalupe Hidalgo officially ended the war. The United States bought New Mexico and California for $15 million, and Mexico recognized the U.S. border at the Rio Grande. The American state of Texas was established—half slave and half free—sixteen years before the country plunged into an even bloodier war.