After Texas attained its independence, the idea of its incorporation into the United States gained support both in Texas and in the United States Congress. Definitive action on the measure was delayed for several years, however, because of the divisive issue of admitting another slave state into the United States and the likely prospect that annexation would provoke a war with Mexico. In early 1845, the United States Congress passed a resolution in favor of the annexation of Texas, which prompted Mexico to sever diplomatic relations with the United States. The Mexican congress had never ratified Santa Anna's secret treaty with the Texans, and to underscore its opposition to Texas's independence, the Mexican congress passed a law that retroactively annulled any treaties signed by a Mexican negotiator while in captivity.

Further aggravating the dispute was the fact that the Texans had issued a dubious territorial claim that expanded the republic's southern and western boundary from the previously accepted Nueces River to the Río Bravo del Norte. By claiming all of the land up to the headwaters of the Río Bravo del Norte, the Texans more than doubled the size of their republic to include parts of present-day New Mexico, Colorado, Oklahoma, Kansas, and all of present-day western Texas.

Shortly after Texas was admitted to the Union as the twenty-eighth state, President James K. Polk dispatched a special envoy, John Slidell, to Mexico City to settle the Texas boundary dispute and to arrange the purchase of California. The Mexican president, José Joaquín Herrera, had been willing to recognize an independent Texas but was under intense domestic pressure to reject United States annexation and Texas's expanded territorial claim. As a result, he refused to meet Slidell and began reinforcing Mexican army units along the Río Bravo del Norte.

Hostilities between Mexico and the United States began on April 25, 1846, when several United States soldiers were killed in a cavalry skirmish with Mexican forces in the disputed territory. Shortly after the two sides declared war, Santa Anna was recalled from exile in Cuba to once again lead Mexican troops against a foreign invasion.

The United States Army attacked on three fronts: one column, under General Stephen W. Kearney, occupied California and New Mexico; another column, under General Zachary Taylor, entered northern Mexico; and a third detachment, commanded by General Winfield Scott, landed at Veracruz and marched to Mexico City. California and New Mexico fell with little bloodshed. Northern Mexico was the scene of fierce battles between Taylor and Santa Anna's armies at Buena Vista. Santa Anna initially struck hard at the outnumbered United States forces, but he later abandoned the battle and returned to Mexico City, prematurely claiming victory.

The heaviest fighting was done by Scott's Army of Occupation, which landed at Veracruz on March 9, 1847. Rather than attempt to occupy the city outright, Scott positioned his forces west of it, cutting off Veracruz's supply line from the capital. After several days of heavy naval bombardment that killed hundreds of civilians, Veracruz surrendered on March 27, 1847.

In Mexico City, the situation was chaotic. President once again, Santa Anna denounced both congress and his own subordinates in the executive branch for their lack of resolve in preparing the defense of the capital. They, in turn, denounced him for his failures in battle. On August 20, 1847, the Army of Occupation asked for the surrender of Mexico City, but the battle continued until September 13, 1847, when the last bastion of Mexican resistance fell during the famous Battle of Chapultepec. During the battle, young cadets from the Mexican military academy, the Niños Héroes (or "boy heroes") leapt to their deaths rather than surrender. The United States victory marked the end of the war and the beginning of negotiations for peace.

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Source: U.S. Library of Congress