Mexican-American War — The Conflict That Shaped the Modern United States

The Mexican-American War was a conflict that transformed the United States. Fought between 1846 and 1848, the war resulted in the American acquisition of vast territories — including California, Arizona, New Mexico, Nevada, Utah, and parts of Colorado and Wyoming — that made the United States a continental nation. The war was also deeply controversial, with critics arguing it was an aggressive war of conquest fought to expand slavery.

The Mexican-American War was the first major American war fought primarily on foreign soil. It was also a war that trained many of the officers who would command the opposing armies in the Civil War — Ulysses S. Grant, Robert E. Lee, Winfield Scott, and many others. The war’s consequences — the addition of vast new territories and the question of whether they would be slave or free — would help trigger the Civil War.

Causes of the War

The immediate cause of the war was the annexation of Texas by the United States in 1845. Texas had won its independence from Mexico in 1836 and had been an independent republic for nearly a decade. The annexation was popular in the United States but was deeply offensive to Mexico, which still considered Texas a breakaway province.

The border between Texas and Mexico was disputed. Texas claimed the Rio Grande as its southern border, while Mexico claimed the Nueces River, about 150 miles to the north. President James K. Polk, an expansionist Democrat who had been elected on a platform of territorial expansion, sent American troops under General Zachary Taylor to the disputed territory between the two rivers.

Polk also wanted to acquire California and New Mexico from Mexico. He offered to purchase these territories, but the Mexican government refused. Polk then sought a war to take the territories by force. The opportunity came in April 1846 when Mexican forces crossed the Rio Grande and attacked an American patrol. Polk asked Congress for a declaration of war, claiming that Mexico had “shed American blood upon the American soil.”

The War in the North

The American strategy called for offensives in three directions: an invasion of northern Mexico by Zachary Taylor, an occupation of New Mexico and California by the Army of the West under Stephen Kearny, and, ultimately, an invasion of central Mexico.

Taylor’s campaign in northern Mexico was marked by a series of victories against larger Mexican armies. At Palo Alto and Resaca de la Palma in May 1846, American artillery and infantry defeated the Mexican forces under General Mariano Arista. Taylor then crossed the Rio Grande and captured the Mexican city of Monterrey after a hard-fought battle in September 1846.

The Battle of Buena Vista in February 1847 was the most dramatic battle of the northern campaign. Taylor faced the Mexican army under General Antonio Lopez de Santa Anna, who had returned from exile to lead the Mexican forces. Santa Anna attacked with about 15,000 men against Taylor’s 5,000. The American army held through a desperate day-long battle, and Santa Anna withdrew.

The Conquest of California and New Mexico

The American conquest of California and New Mexico was accomplished with remarkable speed and minimal resistance. Colonel Stephen Kearny marched from Fort Leavenworth, Kansas, with the Army of the West, about 1,700 men. He occupied Santa Fe, the capital of New Mexico, without resistance in August 1846.

In California, American settlers had already risen against Mexican rule in the Bear Flag Revolt of June 1846. The American navy under Commodore John D. Sloat captured Monterey and San Francisco. Kearny, after a difficult march across the desert, defeated Mexican forces at the Battle of San Pasqual and joined with American naval forces to complete the conquest of California.

The Mexican population of California and New Mexico offered little resistance to American occupation. The American military government that followed was relatively benign, and the territories were quickly pacified.

The Invasion of Central Mexico

The final phase of the war was the American invasion of central Mexico, commanded by General Winfield Scott. Scott, one of the most capable officers in American history, conceived a daring plan: to land at the port of Veracruz and march inland to Mexico City, following the same route that Hernan Cortes had taken three centuries earlier.

Scott’s campaign was a masterpiece of military planning and execution. He landed his army of 10,000 men at Veracruz in March 1847 and besieged the city, which surrendered after three weeks. Scott then began his march toward Mexico City.

The Mexican army under Santa Anna attempted to block Scott’s advance at Cerro Gordo, a mountain pass that appeared to be impregnable. Scott sent a flanking column that discovered a path around the Mexican position, and the Americans attacked from the rear, routing the Mexican army.

Scott continued his advance, fighting and winning a series of battles at Contreras, Churubusco, Molino del Rey, and Chapultepec. The capture of Chapultepec Castle, the last defensive position before Mexico City, was the climactic battle of the war. The American flag was raised over the palace of Montezuma, and Mexico City was occupied.

The Treaty of Guadalupe Hidalgo

The Treaty of Guadalupe Hidalgo, signed in February 1848, ended the war. Mexico recognized the annexation of Texas and ceded California and New Mexico to the United States. The United States paid Mexico $15 million and assumed $3.25 million in claims by American citizens against Mexico.

The Mexican Cession added 529,000 square miles to the United States — about 14 percent of the country’s current land area. The territories acquired included all of modern California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado, Wyoming, Kansas, and Oklahoma.

The war was a disaster for Mexico. The country lost half of its territory, suffered an estimated 50,000 casualties, and experienced a period of political chaos and foreign intervention. The defeat left a legacy of bitterness and distrust toward the United States that persists in some quarters today.

Legacy of the War

The Mexican-American War had profound consequences for the United States. It added vast territories that made the United States a continental nation. The discovery of gold in California in 1848 — just days before the treaty was signed — triggered the California Gold Rush, which transformed the American economy and society.

The war also deepened the sectional divide over slavery. The question of whether the new territories would be slave or free became the central issue of American politics in the 1850s. The Compromise of 1850, the Kansas-Nebraska Act, and the Dred Scott decision were all attempts to resolve this question. The failure of these compromises led to the Civil War.

The Mexican-American War trained a generation of American officers. Ulysses S. Grant, Robert E. Lee, Thomas Jackson, George Meade, and many others served in the war. Grant later wrote that he considered the war “one of the most unjust ever waged by a stronger against a weaker nation.”

The war also established patterns of American military intervention in Latin America. The willingness to use force to achieve territorial and strategic objectives, the assumption of American superiority, and the tendency to underestimate the resistance of local populations all characterized later American interventions.

The Mexican-American War connects to broader themes in American military history. The Civil War that followed was shaped by the experience and leadership of officers who had served in Mexico. The Spanish-American War and later interventions continued the pattern of American expansion.

Frequently Asked Questions

Why did the United States go to war with Mexico?

The United States went to war over the annexation of Texas, a disputed border, and President Polk’s desire to acquire California and New Mexico. Critics argued the war was an aggressive war of conquest.

How much territory did the United States acquire?

The United States acquired 529,000 square miles, including all of modern California, Nevada, Utah, Arizona, New Mexico, and parts of several other states.

Was the Mexican-American War just?

The war was deeply controversial at the time and remains so today. Many Americans, including Ulysses S. Grant, considered it an unjust war of aggression against a weaker neighbor.

How did the war affect the slavery debate?

The acquisition of vast new territories reopened the question of whether slavery would be allowed in the territories, a question that the Missouri Compromise had supposedly settled. The controversy over slavery in the new territories led directly to the Civil War.

Conclusion

The Mexican-American War was a conflict of enormous consequence. It added vast territories to the United States, making the country a continental nation stretching from the Atlantic to the Pacific. The war was also deeply controversial, criticized as an aggressive war of conquest fought to expand slavery. Its legacy — the addition of new territories and the deepening of the slavery controversy — helped trigger the Civil War. Understanding the Mexican-American War is essential for understanding how the United States became the nation it is today.