History Documentary: The Mexican - American War Mexican War (1846 to 1848); Full Documentary: the U.S.–Mexican War, or the Invasion of Mexico

The Mexican–American War, also known as the Mexican War, the U.S.–Mexican War or the Invasion of Mexico, was an armed conflict between the United States of America and the United Mexican States from 1846 to 1848. It followed in the wake of the 1845 US annexation of Texas, which Mexico considered part of its territory, despite the 1836 Texas Revolution.

The Republic of Texas. The present-day outlines of the individual US states are superimposed on the boundaries of 1836–1845.

After its independence in 1821 and brief experiment with monarchy, Mexico became a republic in 1824, characterized by considerable instability, so that when war broke out in 1846, Mexico was ill-prepared for this conflict. The war with the United States followed in the wake of decades of Indian raids in the sparsely settled north of Mexico, which the avenger government-sponsored American migration to the Mexican province of Texas was aimed at buffering. Americans and some Mexicans revolted against the Mexican government in the 1836 Texas Revolution, creating a republic not recognized by Mexico, which still claimed it as its national territory. The 1845 expansion of US territory with its annexation of Texas escalated the dispute between the United States and Mexico to open war.

In 1844 James K. Polk, the newly-elected president, made a proposition to the Mexican government to purchase the disputed lands. When that offer was rejected, troops from the United States were moved into the disputed territory of Coahuila. These troops were then attacked by Mexican troops, killing 12 American troops and taking 52 prisoners. These same Mexican troops later laid siege to a US fort along the Rio Grande. This would lead to the conflict that resulted in the loss of much of Mexico's northern territory.

US forces quickly occupied Santa Fe de Nuevo México and Alta California Territory, then invaded parts of Northeastern Mexico and Northwest Mexico; meanwhile, the Pacific Squadron conducted a blockade, and took control of several garrisons on the Pacific coast farther south in Baja California Territory. Another American army, under the command of General Winfield Scott, captured the capital Mexico City, marching from the port of Veracruz, virtually unopposed.

The 1848 Treaty of Guadalupe Hidalgo ended and specified the major consequence of the war: the Mexican Cession of the territories of Alta California and Santa Fe de Nuevo México to the United States in exchange for $15 million. In addition, the United States assumed $3.25 million of debt owed by the Mexican government to US citizens. Mexico recognized the loss of Texas and thereafter cited the Rio Grande as its national border with the United States.

American territorial expansion to the Pacific coast had been the goal of US President James K. Polk, the leader of the Democratic Party. The war was highly controversial in the United States, with the Whig Party, anti-imperialists and anti-slavery elements strongly opposed. Critics in the United States pointed to heavy casualties of the US forces and high monetary cost of the conflict. The war intensified the slavery issue in the United States, leading to bitter debates that culminated in the bloody American Civil War.

In Mexico, the war came in the middle of political turmoil, which increased into chaos during the conflict. The military defeat and loss of territory was a disastrous blow, causing Mexico to enter "a period of self-examination ... as its leaders sought to identify and address the reasons that had led to such a debacle." In the immediate aftermath of the war, some prominent Mexicans wrote that the war that resulted in "the state of degradation and ruin" in Mexico, and saw for "the true origin of the war, it is sufficient to say that the insatiable ambition of the United States, favored by our weakness, caused it." The shift in the Mexico-U.S. border left many Mexican citizens separated from their national government. For the indigenous peoples who had never accepted Mexican rule, the change in border meant conflicts with a new outside power.