Roots of the Mexican-American War

Major Dix at the Battle of Buena Vista, during the Mexican-American War, Mexico, 23rd February 1847
Kean Collection/Archive Photos/Getty Images

The Mexican-American War (1846 to 1848) was a long, bloody conflict between the United States of America and Mexico. It would be fought from California to Mexico City and many points in between, all of them on Mexican soil. The USA won the war by capturing Mexico City in September of 1847 and forcing the Mexicans to negotiate a truce favorable to US interests.

By 1846, the war was nearly inevitable between the USA and Mexico. On the Mexican side, the lingering resentment over the loss of Texas was intolerable. In 1835, Texas, then part of the Mexican State of Coahuila and Texas, had risen in revolt. After setbacks at the Battle of the Alamo and the Goliad Massacre, the Texan rebels stunned Mexican General Antonio López de Santa Anna at the Battle of San Jacinto on April 21, 1836. Santa Anna was taken prisoner and forced to recognize Texas as an independent nation. Mexico, however, did not accept Santa Anna's agreements and considered Texas nothing more than a rebellious province.

Since 1836, Mexico had half-heartedly tried to invade Texas and take it back, without much success. The Mexican people, however, clamored for their politicians to do something about this outrage. Although privately many Mexican leaders knew that reclaiming Texas was impossible, to say so in public was political suicide. The Mexican politicians outdid each other in their rhetoric saying that Texas must be brought back into Mexico.

Meanwhile, tensions were high on the Texas/Mexico border. In 1842, Santa Anna sent a small army to attack San Antonio: Texas responded by attacking Santa Fe. Not long after, a bunch of Texan hotheads raided the Mexican town of Mier: they were captured and poorly treated until their release. These events and others were reported in the American press and were generally slanted to favor the Texan side. The simmering disdain of Texans for Mexico thus spread to the entire USA.

In 1845, the USA began the process of annexing Texas to the union. This was truly intolerable for Mexicans, who may have been able to accept Texas as a free republic but never part of the United States of America. Through diplomatic channels, Mexico let it be known that to annex Texas was practically a declaration of war. The USA went ahead anyway, which left Mexican politicians in a pinch: they had to do some saber-rattling or look weak.

Meanwhile, the USA had its eye on Mexico's northwestern possessions, such as California and New Mexico. The Americans wanted more land and believed that their country should stretch from the Atlantic to the Pacific. The belief that America should expand to fill the continent was called "Manifest Destiny." This philosophy was expansionist and racist: its proponents believed that the "noble and industrious" Americans deserved those lands more than the "degenerate" Mexicans and Native Americans who lived there.

The USA tried on a couple of occasions to purchase those lands from Mexico and was rebuffed every time. President James K. Polk, however, would not take no for an answer: he meant to have California and Mexico's other western territories and he would go to war to have them.

Fortunately for Polk, the border of Texas was still in question: Mexico claimed it was the Nueces River while the Americans claimed it was the Rio Grande. In early 1846, both sides sent armies to the border: by then, both nations were looking for an excuse to fight. It wasn't long before a series of small skirmishes bloomed into war. The worst of the incidents was the so-called "Thornton Affair" of April 25, 1846, in which a squad of American cavalrymen under the command of Captain Seth Thornton was attacked by a much larger Mexican force: 16 Americans were killed. Because the Mexicans were in contested territory, President Polk was able to ask for a declaration of war because Mexico had "…shed American blood upon the American soil." Larger battles followed within two weeks and both nations had declared war on one another by May 13.

The war would last about two years, until the spring of 1848. The Mexicans and Americans would fight about ten major battles, and the Americans would win all of them. In the end, the Americans would capture and occupy Mexico City and dictate terms of the peace agreement to Mexico. Polk got his lands: according to the Treaty of Guadalupe Hidalgo, formalized in May of 1848, Mexico would hand over most of the current US Southwest (the border established by the treaty is very similar to today's border between the two nations) in exchange for $15 million dollars and forgiveness of some previous debt.


  • Brands, H.W. Lone Star Nation: the Epic Story of the Battle for Texas Independence. New York: Anchor Books, 2004.
  • Eisenhower, John S.D. So Far from God: the U.S. War with Mexico, 1846-1848. Norman: the University of Oklahoma Press, 1989
  • Henderson, Timothy J. A Glorious Defeat: Mexico and its War with the United States.New York: Hill and Wang, 2007.
  • Wheelan, Joseph. Invading Mexico: America's Continental Dream and the Mexican War, 1846-1848. New York: Carroll and Graf, 2007.
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Minster, Christopher. "Roots of the Mexican-American War." ThoughtCo, Apr. 5, 2023, Minster, Christopher. (2023, April 5). Roots of the Mexican-American War. Retrieved from Minster, Christopher. "Roots of the Mexican-American War." ThoughtCo. (accessed June 10, 2023).