Mexican-American War (1846-1848): War for the American West

The Mexican-American War was fought between the United States and Mexico from April 1846 to February 1848. It grew out of the US annexation of Texas and President James K. Polk’s ambition to acquire California and the Southwest. American armies won every major battle and captured Mexico City, and in the Treaty of Guadalupe Hidalgo Mexico gave up about half of its territory, including Texas. The war made the United States a continental power stretching to the Pacific, but it also deepened the conflict over slavery that led to the American Civil War.

Background and causes

Mexico won independence from Spain in 1821 but struggled with political instability, frequent coups and debt. Its northern territories, from Texas to California, were thinly settled. Texas broke away in 1836 and was annexed by the United States in 1845 (see Annexation of Texas), which Mexico regarded as theft of its province. Mexico broke off diplomatic relations in March 1845.

Expansionism was a powerful force in the United States. In 1845 the journalist John L. O’Sullivan wrote of America’s “manifest destiny to overspread the continent”, a phrase that came to sum up the belief that the country was destined to spread across North America. Polk, elected in 1844 on an expansionist platform, wanted not only Texas but also California, with its fine Pacific harbours, and New Mexico.

Two disputes brought the crisis to a head. The first was the border: Texas claimed the Rio Grande, while Mexico regarded the Nueces River, about 150 miles further north, as the limit. The second was money: US citizens had claims against Mexico for damages, which Mexico could not pay. In late 1845 Polk sent John Slidell to Mexico City to offer as much as 25 to 30 million dollars for California and New Mexico and to settle the border. The Mexican government, under strong nationalist pressure, refused to receive him.

Outbreak of war

In January 1846 Polk ordered General Zachary Taylor to advance from Corpus Christi to the Rio Grande. Taylor built a fort opposite the Mexican town of Matamoros. On 25 April 1846 Mexican cavalry attacked a US patrol of about 70 to 80 dragoons under Captain Seth Thornton north of the river, killing about a dozen and capturing most of the rest, an incident known as the Thornton Affair.

Polk sent a war message to Congress on 11 May, declaring that Mexico had “shed American blood upon the American soil.” Congress declared war on 13 May 1846 by large majorities, though many Whigs voted for it only to support the troops. Before the news even reached him, Taylor had already defeated Mexican forces at Palo Alto on 8 May and Resaca de la Palma on 9 May, thanks largely to his mobile “flying artillery”.

The northern campaign

Taylor crossed the Rio Grande and marched into northern Mexico. After hard street fighting he captured the city of Monterrey on 24 September 1846, then agreed a controversial eight-week armistice that angered Polk.

Meanwhile Antonio López de Santa Anna, Mexico’s most famous general, returned from exile in Cuba. Polk had allowed him through the US naval blockade, believing Santa Anna would negotiate peace; instead he took command of the Mexican army. In February 1847, with about 15,000 men, he attacked Taylor’s much smaller force of about 4,600, mostly volunteers, at Buena Vista near Saltillo. After two days of fierce fighting on 22–23 February, Santa Anna withdrew. The battle made Taylor a national hero, and he was elected president in 1848.

New Mexico and California

In the west the United States moved quickly to seize the lands it wanted. General Stephen W. Kearny’s Army of the West marched from Fort Leavenworth and occupied Santa Fe without a fight on 18 August 1846. A revolt by Hispano and Pueblo residents at Taos in January 1847 was suppressed, and its leaders were executed.

In California, American settlers near Sonoma proclaimed the short-lived Bear Flag Republic in June 1846, backed by the army explorer John C. Frémont. Commodore John D. Sloat’s naval forces raised the US flag at Monterey on 7 July 1846. Californios, the Mexican residents of California, rose up in the autumn and defeated Kearny’s troops at San Pasqual in December, but US forces retook Los Angeles in January 1847. The Treaty of Cahuenga on 13 January 1847 ended the fighting in California. Colonel Alexander Doniphan’s Missouri volunteers, meanwhile, marched deep into Chihuahua.

Scott’s march on Mexico City

Mexico would not make peace despite losing its northern lands, so Polk approved a direct strike at the capital. General Winfield Scott landed about 10,000 troops near Veracruz on 9 March 1847, the largest amphibious landing in US history until then. After a siege and bombardment, the city surrendered on 29 March.

Scott then advanced inland along the route Hernán Cortés had taken three centuries earlier. He outflanked and routed Santa Anna at Cerro Gordo on 18 April 1847, with the young engineer Captain Robert E. Lee finding a path around the Mexican position. The army waited at Puebla through the summer for reinforcements.

In August Scott moved on the Valley of Mexico. He won the battles of Contreras and Churubusco on 19–20 August. At Churubusco US troops faced the San Patricio Battalion, made up largely of Irish and other European immigrants who had deserted from the US Army; many of those captured were later hanged as deserters. After a failed armistice, Scott took Molino del Rey on 8 September and stormed Chapultepec Castle, home of the national military academy, on 13 September 1847. US troops entered Mexico City the next day, and Santa Anna resigned the presidency.

The peace treaty

The State Department official Nicholas Trist had accompanied Scott’s army to negotiate peace. Polk recalled him in late 1847, but Trist stayed on and signed the Treaty of Guadalupe Hidalgo on 2 February 1848. Mexico recognised the Rio Grande as the border of Texas and ceded Alta California and New Mexico, in return for 15 million dollars and the US assumption of up to 3.25 million dollars in claims. The Senate ratified the treaty on 10 March 1848, and the last American troops left Mexico City in June.

Opposition and controversy

The war divided Americans. Many Whigs saw it as an unjust war of conquest started by Polk. The young Illinois congressman Abraham Lincoln introduced his Spot Resolutions in December 1847, and Henry David Thoreau, jailed for refusing to pay a tax he linked to slavery and the war, wrote the essay later known as Civil Disobedience. Years later Ulysses S. Grant, who fought in the war as a lieutenant, called it in his memoirs one of the most unjust wars ever waged by a stronger nation against a weaker one.

The fiercest dispute was over slavery. In August 1846 Congressman David Wilmot proposed that slavery be banned in any territory taken from Mexico. The Wilmot Proviso passed the House several times but always failed in the Senate, and the question of slavery in the new lands dominated politics until the Compromise of 1850.

By the numbers

Consequences and legacy

For the United States the war completed a sweep of expansion that, with the Oregon Treaty of 1846, made it a nation spanning the continent. Gold was found in California just days before the treaty was signed, setting off the California Gold Rush. Many officers who learned their trade in Mexico, including Grant, Lee, Thomas “Stonewall” Jackson and George McClellan, later led armies against each other in the Civil War.

For Mexico the defeat was a national trauma. Mexicans call it the Intervención estadounidense, the US intervention, and remember the Niños Héroes, cadets who died defending Chapultepec. The loss fed years of political turmoil, including the Reform War of 1857–1861, and left a lasting mistrust of its northern neighbour. Tens of thousands of Mexican citizens in the ceded lands became Americans almost overnight, the foundation of today’s Mexican American communities in the Southwest.

Timeline

• 1845, 29 December: Texas becomes a US state.
• 1846, 25 April: Thornton Affair on the north bank of the Rio Grande.
• 1846, 8-9 May: US victories at Palo Alto and Resaca de la Palma.
• 1846, 13 May: The United States declares war on Mexico.
• 1846, 18 August: Kearny occupies Santa Fe.
• 1846, 24 September: Taylor captures Monterrey.
• 1847, 22-23 February: Battle of Buena Vista.
• 1847, 29 March: Veracruz surrenders to Scott.
• 1847, 13-14 September: Chapultepec is stormed; US troops enter Mexico City.
• 1848, 2 February: Treaty of Guadalupe Hidalgo signed.

Why it matters

The Mexican-American War redrew the map of North America, giving the United States California and the Southwest and costing Mexico roughly half of its territory. It shaped the identity of the American West and the borderlands, left a deep wound in Mexican national memory and reopened the question of slavery’s expansion that would tear the United States apart in 1861.