The Mexican-American War

Map illustrating the territories ceded by Mexico to the U.S. in 1848

Kballen / Wikimedia Commons / CC-BY-SA-3.0

From 1846 to 1848, the United States of America and Mexico went to war. There were several reasons why they did so, but the most important ones were the U.S. annexation of Texas and the Americans' desire for California and other Mexican territories. The Americans took the offensive, invading Mexico on three fronts: from the north through Texas, from the east through the port of Veracruz, and into the west (present-day California and New Mexico). The Americans won every major battle of the war, mostly thanks to superior artillery and officers. In September 1847, American General Winfield Scott captured Mexico City. This was the final straw for the Mexicans, who finally sat down to negotiate. The war was disastrous for Mexico, as it was forced to sign away nearly half of its national territory, including California, New Mexico, Nevada, Utah, and parts of several other current U.S. states.

The Western War

American President James K. Polk intended to invade and hold the territories he wanted, so he sent General Stephen Kearny west from Fort Leavenworth with 1,700 men to invade and hold New Mexico and California. Kearny captured Santa Fe and then divided his forces, sending a large contingent south under Alexander Doniphan. Doniphan would eventually take the city of Chihuahua.

Meanwhile, the war had already begun in California. Captain John C. Frémont had been in the region with 60 men; they organized American settlers in California to revolt against the Mexican authorities there. He had the support of some U.S. navy vessels in the area. The struggle between these men and the Mexicans went back and forth for a few months until Kearny arrived with what was left of his army. Although he was down to fewer than 200 men, Kearny made the difference; by January of 1847, the Mexican northwest was in American hands.

General Taylor's Invasion

American General Zachary Taylor was already in Texas with his army waiting for hostilities to break out. There was already a large Mexican army on the border as well; Taylor routed it twice in early May of 1846 at the Battle of Palo Alto and the Battle of Resaca de la Palma. During both battles, superior American artillery units proved the difference.

The losses forced the Mexicans to retreat to Monterrey. Taylor followed and took the city in September of 1846. Taylor moved to the south and was engaged by a massive Mexican army under the command of General Santa Anna at the Battle of Buena Vista on February 23, 1847. Taylor once again prevailed.

The Americans hoped that they had proved their point. Taylor's invasion had gone well and California was already securely under control. They sent envoys to Mexico in the hopes of ending the war and gaining the land they desired, but Mexico would have none of it. Polk and his advisors decided to send yet another army into Mexico and General Winfield Scott was selected to lead it.

General Scott's Invasion

The best route to get to Mexico City was to go through the Atlantic port of Veracruz. In March of 1847, Scott began landing his troops near Veracruz. After a short siege, the city surrendered. Scott marched inland, defeating Santa Anna at the Battle of Cerro Gordo on April 17-18 along the way. By August Scott was at the gates of Mexico City itself. He defeated the Mexicans at the Battles of Contreras and Churubusco on August 20, gaining a toehold into the city. The two sides agreed to a brief armistice, during which time Scott hoped the Mexicans would finally negotiate, but Mexico still refused to sign away its territories to the north.

In September of 1847, Scott attacked once again, crushing the Mexican fortification at Molino del Rey before assaulting the Chapultepec Fortress, which was also the Mexican Military Academy. Chapultepec guarded the entrance to the city; once it fell the Americans were able to take and hold Mexico City. General Santa Anna, seeing that the city had fallen, retreated with what troops he had left to unsuccessfully try and cut the American supply lines near Puebla. The major combat phase of the war had ended.

The Treaty of Guadalupe Hidalgo

The Mexican politicians and diplomats were finally forced to negotiate in earnest. For the next few months, they met with American diplomat Nicholas Trist, who had been ordered by Polk to secure all of the Mexican northwest in any peace settlement.

In February of 1848, the two sides agreed on the Treaty of Guadalupe Hidalgo. Mexico was forced to sign over all of California, Utah, and Nevada as well as parts of New Mexico, Arizona, Wyoming, and Colorado in exchange for $15 million dollars and exoneration of about $3 million more in previous liability. The Rio Grande was established as the border of Texas. People living in these territories, including several Indigenous groups, reserved their properties and rights and were to be given U.S. citizenship after a year. Lastly, future disagreements between the U.S. and Mexico would be settled by mediation, not warfare.

Legacy of the Mexican-American War

Although it is often overlooked in comparison with the American Civil War, which broke out about 12 years later, the Mexican-American War was just as important to American History. The massive territories gained during the war make up a large percentage of present-day United States. As an added bonus, gold was discovered shortly thereafter in California which made the newly acquired lands even more valuable.

The Mexican-American War was in many ways a precursor to the Civil War. Most of the important Civil War generals fought in the Mexican-American War, including Robert E. Lee, Ulysses S. Grant, William Tecumseh Sherman, George Meade, George McClellan, and Stonewall Jackson. The tension between the pro-slavery states of the southern U.S. and the anti-slavery states of the north was made worse by the addition of so much new territory; this hastened the onset of the Civil War.

The Mexican-American War made the reputations of future U.S. presidents. Ulysses S. Grant, Zachary Taylor, and Franklin Pierce all fought in the war, and James Buchanan was Polk's Secretary of State during the war. A Congressman named Abraham Lincoln made a name for himself in Washington by vocally opposing the war. Jefferson Davis, who would become president of the Confederate States of America, also distinguished himself during the war.

If the war was a bonanza for the United States of America, it was a disaster for Mexico. If Texas is included, Mexico lost more than half of its national territory to the U.S. between 1836 and 1848. After the bloody war, Mexico was in ruins physically, economically, politically, and socially. Many peasant groups took advantage of the chaos of war to lead uprisings all over the country; the worst was in Yucatan, where hundreds of thousands of people were killed.

Although Americans have forgotten about the war, for the most part, many Mexicans are still irate about the "theft" of so much land and the humiliation of the Treaty of Guadalupe Hidalgo. Even though there is no realistic chance of Mexico ever reclaiming those lands, many Mexicans feel they still belong to them.

Because of the war, there was much bad blood between the U.S. and Mexico for decades. Relations did not begin to improve until World War II when Mexico decided to join the Allies and make common cause with the U.S.


  • 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. "The Mexican-American War." ThoughtCo, Apr. 5, 2023, Minster, Christopher. (2023, April 5). The Mexican-American War. Retrieved from Minster, Christopher. "The Mexican-American War." ThoughtCo. (accessed June 1, 2023).