The Mexican-American War

Color lithograph of a battlefield with forces exchanging fire with a fortress on a hill above
The Battle of Palo Alto was the first conflict of the Mexican-American War

Quick Facts
Matamoros, Mexico
First Battle of Mexican-American War
National Historical Park
On May 13, 1846, the United States declared war on Mexico, beginning the Mexican-American War. The Mexican-American War is one of the least known pivotal moments in US History. It paved the way for so many other important events, from the expansion and dispossession of indigenous people, the California Gold Rush, and American Civil War. It added the states of California, Texas, New Mexico, Utah, Arizona, Nevada, and parts of Colorado and Wyoming to the United States. American success in the war solidified belief in the concept of Manifest Destiny, confirming the idea that the United States had been destined by divine entities to expand into a continental empire resembling the present-day nation. For proponents of slavery and abolition, the new territory provided a source of conflict over whether slavery would expand and continue in the West. For the major military leaders of the Civil War on both the Confederate and Union sides, including such Pig War figures as Winfield Scott, George Pickett, and William Harney, it was a crucible in which their skills were tested, their tactics were formed, and they built the bonds of camaraderie that they relied on during the Civil War.

The immediate cause of the Mexican-American War was a disputed boundary between the United States and Texas on the Nueces Strip. Mexico did not recognize Texas as legitimate American territory and Texas admission to the United States antagonized Mexican officials and citizens. Rather than ameliorate this problem, President Polk intentionally worked to aggravate Mexico and provoke a war. On January 13, 1846, Polk ordered American forces into deeply disputed territory. In April, an army of approximately 4,000 men lead by General Zachary Taylor entered the Nueces Strip, a contested territory that Mexico and many Americans regarded as never having been a part of Texas. Polk knew this action would antagonize Mexican military forces stationed within sight of Taylor’s army at Matamoros. Colonel Hitchcock, who served with Zachary Taylor’s army, could see the real intention of his deployment from his vantage point on the front lines: “We have not one particle of right to be here. It looks as if the government sent a small force on purpose to bring on a war, so as to have a pretext for taking California and as much of this country as it chooses.” 

On April 9, Taylor’s army deployed the bulk of its artillery within striking distance of the 16,000 person city of Matamoros, Mexico. The US Navy then cut Matamoros off from the Gulf of Mexico, creating a blockade by land and water of the Mexican civilians and soldiers just across the Nueces River from Taylor’s forces. On April 24, Mexican forces crossed the Matamoros River to break the blockade; Taylor sent a small force to respond, and the Mexican Army opened fire, killing 11 American soldiers. The site of this battlefield is now preserved as Palo Alto Battlefield National Historical Park.

When word reached Washington, D.C., two weeks later, President James K. Polk informed congress that: "The cup of forbearance had been exhausted even before the recent information from the frontier of the Del Norte [Rio Grande]. But now, after reiterated menaces, Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil. She has proclaimed that hostilities have commenced, and that the two nations are now at war.

Prior to these actions, America was deeply divided about the prospect of war with Mexico. Abraham Lincoln, then a US Congressman tried to challenge the war by demanding proof that the spot where the war began was on US soil. Transcendentalist Henry David Thoreau engaged in civil disobedience, going to jail because he refused to pay federal taxes that supported the war."

Most Americans, however rallied around the flag. Many individuals enlisted in the army, swelling the small US army with new recruits. These recruits were motivated by more than just patriotic feelings. As American forces rapidly defeated Mexican forces, sexual assaults, looting, and violence toward civilians and surrendered Mexican forces were common. At the highest level, Commanding General Winfield Scott forced Mexican authorities to pay massive ransoms that supported underfunded American troops in the field. Historian Amy Greenberg has also shown how racist attitudes that saw Mexicans as racial inferiors and anti-Catholic bigotry enabled American soldiers and leaders to justify extreme violence and what we would now regard as war crimes against Mexican forces and civilians.

The US military was wildly successful and used a combination of innovative strategy, competent officership, luck, and brutality to achieve victory. American forces captured Mexico City within 16 months of the war’s beginning and expeditionary forces also fought battles in California and New Mexico that enabled the seizure of vast tracts of territory. As the Mexican-American War came to an end, gold was discovered in California, causing a massive population surge on the West Coast that ensured continued American sovereignty over the entire West Coast and paved the way for the boundary conflict in the San Juan Islands.

San Juan Island National Historical Park

Last updated: August 4, 2022