The 1846–48 Mexican War redrew the political map of North America, effectively destroying Mexico as a powerful nation and bringing California and the Southwest into the United States. To many contemporaries the conflict seemed a justifiable expression of American “Manifest Destiny.” Modern commentators have been less kind. In her 2012 book, A Wicked War: Polk, Clay, Lincoln and the 1846 U.S. Invasion of Mexico, historian Amy Greenberg denounced the war as “an act of expansionist aggression.” Without question the results, both good and bad, were decisive.
It all began with Texas. The United States had sought to acquire the territory as early as the John Quincy Adams administration of 1825–29. Mexico rebuffed all purchase offers, but whites settled Texas in increasingly large numbers before the fall of the Alamo in March 1836. Outraged Texians then declared independence and went on to defeat General Antonio López de Santa Anna’s Mexican army at the Battle of San Jacinto on April 21. Santa Anna signed the Treaty of Velasco, promising to persuade his government to recognize Texas’ independence, but the Mexican government repudiated the treaty.
For Texians of American extraction, the only means of settling the issue seemed to be annexation by the United States. In 1844 Democratic presidential candidate James K. Polk campaigned on a promise to annex Texas and the land west as far as California. Newspaperman John L. O’Sullivan reflected Polk’s ambitions, shared by many Americans, by coining the term “Manifest Destiny.” Whig Henry Clay was more cautious, however, echoing Northern concerns that Texas would enter the union as a slave state and tip the congressional balance against free states.
Polk narrowly won the ensuing election. Outgoing president John Tyler, interpreting the results as an endorsement of annexation, signed a congressional resolution on March 1, 1845, to make Texas part of the United States. Texians joyfully welcomed annexation. Polk, brushing aside the angry Mexican reaction, sent emissary John M. Slidell to Mexico City offering $25 million (though he was willing to pay up to $30 million) to buy California and another $5 million to buy New Mexico, and seeking to formally establish the Texas border on the Rio Grande.
Polk assumed debt-ridden Mexico would accept Slidell’s offer, but just in case, he initiated secret plans to send white settlers to incite rebellion in the region. Acquiring the territory would inaugurate the United States as a Pacific power, enabling it to join in the burgeoning China trade and challenge Britain’s position in the Northwest. Polk also feared—with due regard— that inaction would encourage British and French economic and political designs on California and Texas.
Mexican President José Joaquín de Herrera had recently taken office and was in a delicate position. He and Polk both realized that continued intransigence would lead to war but feared to back down lest they court disfavor at home. Mexico refused to sell an inch of territory, but Polk pushed on, ignoring warnings of the political turmoil over slavery that would inevitably result.
Texas became the 28th state on Dec. 29, 1845. Mexico, after another coup that overthrew Herrera in favor of the aggressively anti-American General Mariano Paredes y Arrillaga, threatened war. Polk, arguing that Mexico’s rebuff of Slidell provided a pretext for more forceful measures, ordered General Zachary Taylor to march his Army of Occupation to the Rio Grande. This was a provocative act, since Mexico insisted its northern border lay farther north along the Nueces River.
War had by this time become inevitable, and indeed it was popular on both sides of the border. The Americans confidently anticipated victory, while the Mexicans—imagining the United States was too internally divided to fight effectively and that Great Britain might intervene in favor of Mexico—also expected to win the war. Mexican and American troops first clashed on April 25. After the battles of Palo Alto and Resaca de la Palma —both decisive American victories—the United States declared war on May 13, and Mexico followed suit.
The Treaty of Guadalupe Hidalgo, which formally ended the war in February 1848, met all major American demands and provided a bonanza of new territory that carried Manifest Destiny to the Pacific. The consequences were both grand and catastrophic. While the conflict took the United States another step along the road to global power, it also set the stage for the Civil War by pushing the brittle American compromise over slavery toward open dispute. For Mexico the war resulted in the loss of 525,000 square miles of territory (not counting Texas) and wrecked the nation’s already fragile political system. And it may be that for some 21st century Mexicans, immigrating to the United States marks a return to the land of their ancestors.
Originally published in the July 2013 issue of Military History. To subscribe, click here.