The Mexican–American War, fought between 1846 and 1848, reshaped the maps of North America and redefined the destiny of both nations involved. For Mexico, it was a devastating loss that cost nearly half its territory. For the United States, it was a victory that fulfilled the vision of “Manifest Destiny” but also deepened divisions that would later lead to the Civil War. Understanding how this war began, unfolded, and concluded reveals much about the politics, ambitions, and conflicts of the 19th century.
Background and Causes of the War
1. Texas Independence and Annexation
The roots of the conflict stretch back to the 1830s. Texas, then part of Mexico, attracted large numbers of American settlers. These settlers often resisted Mexican laws, particularly those limiting slavery. In 1836, after the famous battles of the Alamo and San Jacinto, Texas declared independence.
Mexico refused to recognize this independence, insisting Texas remained its territory. For nearly a decade, the Republic of Texas existed in a tense limbo, with Mexico threatening to reclaim it and Texans seeking U.S. annexation. When the United States annexed Texas in 1845, Mexico saw it as an act of aggression.
2. Disputed Borders
Even beyond annexation, the two countries disagreed over Texas’s southern boundary. The United States claimed the Rio Grande River as the border, while Mexico insisted it was the Nueces River, farther north. The land between the two rivers became a disputed zone, and it was there that the first shots of the war would be fired.
3. Manifest Destiny
A powerful ideology called Manifest Destiny influenced American politics in the 1840s. Many U.S. leaders believed it was the nation’s God-given mission to expand across the continent, spreading democracy and civilization from the Atlantic to the Pacific. Expansionists saw California and New Mexico—then Mexican territories—as natural additions to the United States. The desire for westward expansion made conflict with Mexico almost inevitable.
4. Diplomatic Failure
In late 1845, President James K. Polk, an ardent expansionist, sent diplomat John Slidell to Mexico to negotiate. Slidell offered to purchase California and New Mexico and settle the boundary dispute. Mexican leaders, facing internal turmoil and unwilling to sell their northern lands, refused to receive him. Polk took this as a deliberate insult.
5. Military Confrontation
Polk ordered General Zachary Taylor to move U.S. troops into the disputed zone between the Nueces and Rio Grande. In April 1846, Mexican cavalry clashed with Taylor’s soldiers, killing several. Polk told Congress that Mexico had “shed American blood upon American soil.” On May 13, 1846, Congress declared war.
The Course of the War
1. Fighting in Northern Mexico
Taylor’s forces scored several early victories in northern Mexico. At the Battle of Palo Alto and Resaca de la Palma in May 1846, U.S. troops pushed back Mexican forces. Later that year, Taylor captured the city of Monterrey after fierce urban fighting.
In February 1847, at the Battle of Buena Vista, Taylor’s smaller force held off a much larger Mexican army led by General Antonio López de Santa Anna, who had returned to power. Buena Vista secured Taylor’s reputation and made him a national hero, eventually propelling him to the presidency.
2. The War in the West
While Taylor fought in northern Mexico, other American forces struck westward. General Stephen W. Kearny marched into New Mexico, taking Santa Fe without a fight. In California, American settlers had already launched the Bear Flag Revolt in June 1846, declaring California an independent republic. Soon after, U.S. naval forces secured California’s coast, ensuring American control.
3. The Invasion of Central Mexico
The most decisive campaign came under General Winfield Scott, who launched an amphibious invasion at Veracruz in March 1847. This was the first large-scale amphibious landing in U.S. history. After capturing Veracruz, Scott marched inland toward Mexico City.
Along the way, his army fought several battles—Cerro Gordo, Contreras, Churubusco, and Molino del Rey—defeating Mexican forces repeatedly. Finally, in September 1847, Scott captured Chapultepec Castle, the last stronghold defending Mexico City. On September 14, American troops entered the Mexican capital, effectively ending the war.
4. Resistance and Heroism in Mexico
Mexican resistance was fierce, even in defeat. At Chapultepec, young military cadets, remembered as Los Niños Héroes (“the boy heroes”), fought to the death rather than surrender. For Mexicans, their bravery became a symbol of sacrifice and national pride in the face of overwhelming odds.
Consequences of the War
1. The Treaty of Guadalupe Hidalgo
The war formally ended with the Treaty of Guadalupe Hidalgo, signed on February 2, 1848. Its terms were harsh for Mexico:
- Mexico recognized the Rio Grande as the Texas border.
- Mexico ceded nearly half its territory to the United States, including California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado and Wyoming.
- The United States paid Mexico $15 million and assumed some claims of American citizens against Mexico.
This vast transfer of land is often called the Mexican Cession.
2. Impact on Mexico
For Mexico, the war was a national tragedy. It lost about 500,000 square miles—almost half its territory. The defeat left Mexico politically unstable and deeply resentful of its powerful northern neighbor. To this day, the war remains a sensitive subject in Mexican memory and identity.
3. Impact on the United States
For the United States, the war was a triumph of expansionism. It fulfilled Polk’s dream of a nation stretching from coast to coast. However, the new lands raised urgent questions: Would slavery be allowed in these territories? This debate inflamed sectional tensions and set the stage for the Civil War.
4. Military and Political Legacies
The war provided valuable experience for a generation of U.S. officers—men like Ulysses S. Grant, Robert E. Lee, and William T. Sherman—who would later fight in the Civil War. Politically, it propelled Zachary Taylor and Winfield Scott to national fame.
For Mexico, Santa Anna’s repeated failures discredited him, though he would continue to reappear in politics.
Leaders’ Words and Reflections
- James K. Polk justified the war by saying: “The cup of forbearance has been exhausted. Mexico has passed the boundary of the United States, has invaded our territory and shed American blood.” His words framed the conflict as defensive, though many saw it as a war of conquest.
- Abraham Lincoln, then a young congressman, criticized Polk’s claims, asking, “Show me the spot where American blood was shed on American soil.” Lincoln’s “Spot Resolutions” questioned whether the clash truly occurred on U.S. territory.
- Ulysses S. Grant, who fought as a young officer, later reflected: “I do not think there was ever a more wicked war than that waged by the United States on Mexico.” His candid assessment highlighted the moral unease surrounding the conflict.
- In Mexico, survivors and historians remembered Chapultepec with reverence. The story of the Niños Héroes became a cornerstone of Mexican national pride.