A War That Redrew the Map
The Mexican-American War was fought from April 1846 to February 1848 between the United States and Mexico. It began with a clash between cavalry patrols in the disputed land between the Nueces River and the Rio Grande in South Texas, and it ended with U.S. troops occupying Mexico City. It is one of the most consequential and least remembered chapters in the story of the United States.
The war grew out of the U.S. annexation of Texas in 1845. Mexico had never recognized Texas's independence, and the two countries disagreed about where Texas even ended: Texas claimed the Rio Grande as its border, while Mexico held that the old boundary was the Nueces River, about 150 miles to the north. President James K. Polk also wanted California and New Mexico, and he tried to buy them before the fighting began.
U.S. forces fought on several fronts at once: Zachary Taylor's army in northern Mexico, Stephen W. Kearny's march to Santa Fe and on to California, naval landings on the Pacific coast, and finally Winfield Scott's amphibious landing at Veracruz and his campaign inland to the Mexican capital. The United States won nearly every major battle, but disease killed far more American soldiers than combat did.
The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, gave the United States roughly 525,000 square miles, including all or part of present-day California, Nevada, Utah, Arizona, New Mexico, Colorado, and Wyoming, in exchange for $15 million. Counting Texas, Mexico lost about half of its territory. The new land reopened the fight over slavery in the United States, and many historians see the war as a step on the road to the Civil War.
At a Glance
1846 to 1848
Fought from April 25, 1846, to February 2, 1848
78,718
U.S. service members who took part (VA)
13,283
U.S. military deaths, most from disease (VA)
525,000 sq mi
Land Mexico ceded in 1848 (National Archives)
The Land That Changed Hands
The pink area marked "Mexican Cession 1848" is the land Mexico gave up under the Treaty of Guadalupe Hidalgo. The blue-green "Texas Annexation 1845" area is the land whose border started the war, and the small orange strip along the southern edge is the 1853 Gadsden Purchase.
Did You Know?
One of Santa Anna's artificial legs, captured at Cerro Gordo in 1847, is on display at the Illinois State Military Museum in Springfield.
It was among the first U.S. wars covered by newspaper war correspondents, and reports reached the East faster than ever thanks to pony express riders, steamships, and the new telegraph, an early chapter in the story of the free press.
About 10,000 U.S. troops landed at Veracruz on March 9, 1847, in a single day without losing a man in the landing, the largest American amphibious landing up to that time.
Disease killed about 6.7 U.S. soldiers for every one killed in battle, according to Department of Veterans Affairs figures.
Gold was found in California on January 24, 1848, nine days before the treaty that made California American was signed.
Two future presidents, Zachary Taylor and Franklin Pierce, were generals in the war, and Ulysses S. Grant served as a young lieutenant.
The U.S. diplomat who negotiated the peace, Nicholas Trist, had already been fired and ordered home when he signed the treaty.
Mexico lost about half of its territory, counting Texas.
The Road to War
No single event caused the war. Texas independence, annexation, a disputed border, unpaid debts, and the drive to reach the Pacific all pushed the two countries toward conflict.
After Mexico won independence from Spain in 1821, it invited American settlers into its thinly populated province of Texas, which was then joined to the state of Coahuila. Empresarios such as Stephen F. Austin brought in thousands of families, and by the early 1830s English-speaking settlers and the enslaved people they brought with them far outnumbered Mexican Tejanos in the region.
Tension grew as Mexico tried to tighten control. An 1830 law restricted new immigration from the United States, Mexico had abolished slavery in 1829 (Texas received a temporary exemption), and in 1835 President Antonio López de Santa Anna set aside the federal Constitution of 1824 in favor of a centralized government. Texans and several Mexican states rebelled against the change.
Texas declared independence on March 2, 1836. Santa Anna's army destroyed the defenders of the Alamo on March 6 and executed several hundred prisoners at Goliad later that month. On April 21, 1836, Sam Houston's army surprised and defeated Santa Anna at San Jacinto and captured him the next day.
As a prisoner, Santa Anna signed the Treaties of Velasco, agreeing to withdraw Mexican troops. The Mexican government rejected those treaties because they were signed under duress, and it continued to regard Texas as a rebel province for the next nine years. That refusal to recognize Texas is the root of everything that followed.
The Republic of Texas asked to join the United States almost at once, but annexation stalled for years. Presidents Andrew Jackson and Martin Van Buren held back, worried about war with Mexico and about adding a large new slave state. A treaty of annexation negotiated under President John Tyler was rejected by the Senate in June 1844 by a vote of 35 to 16.
Annexation became the central issue of the 1844 presidential election. James K. Polk, a Democrat from Tennessee, ran openly in favor of taking Texas and the Oregon Country and defeated the Whig candidate, Henry Clay, who had opposed immediate annexation. Outgoing President Tyler read the result as a mandate.
Instead of a treaty, which needed two-thirds of the Senate, Congress passed a joint resolution of annexation that needed only a simple majority in each chamber. Tyler signed it on March 1, 1845, three days before leaving office. Mexico's minister in Washington, Juan Almonte, protested and left the country, and Mexico broke off diplomatic relations.
A Texas convention accepted the offer on July 4, 1845, and Texas became the 28th state on December 29, 1845. To Mexico, the United States had taken part of its territory. To most Americans who supported annexation, the United States had admitted an independent republic that had asked to join.
Even if annexation were accepted, the two sides disagreed about where Texas ended. Under Spanish and Mexican rule, the province of Texas had stopped at the Nueces River, which reaches the Gulf of Mexico at Corpus Christi. The land south of it, down to the Rio Grande, belonged to the neighboring state of Tamaulipas.
The Republic of Texas claimed the Rio Grande as its southern and western border in an 1836 law, a line that would have included half of present-day New Mexico and the old Mexican town of Santa Fe. Texas never actually governed most of that land, and a Texan expedition sent to take Santa Fe in 1841 was captured by Mexican forces.
The United States adopted the Texas claim. In January 1846 Polk ordered General Zachary Taylor to move his army from Corpus Christi to the Rio Grande. Taylor reached the river opposite the Mexican town of Matamoros on March 28, 1846, and built an earthen fort there, later named Fort Brown.
Supporters of Polk said the army was only defending the border of an American state. Critics at the time, including many Whigs in Congress, said the army had been sent into land that was at best disputed and that the move was likely to start a war. Historians still debate how far Polk meant to provoke a fight, but nearly all agree the land between the rivers was genuinely contested.
In 1845 the New York editor John L. O'Sullivan wrote that it was "our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions." The phrase "Manifest Destiny" caught on as a name for the popular belief that the United States was meant to stretch to the Pacific.
Polk shared that goal. Soon after taking office he told his Secretary of the Navy, George Bancroft, that acquiring California was one of the great aims of his presidency. California's harbors, especially San Francisco Bay, promised trade with Asia, and U.S. officials worried that Britain or France might take the province from a weak Mexican government.
Mexico's hold on its far north was thin. Alta California had only a few thousand Mexican settlers, known as Californios, and they had repeatedly revolted against governors sent from Mexico City. New Mexico had a larger population but was more than 1,000 miles from the capital and depended on trade with Missouri along the Santa Fe Trail.
Expansion was not only a western dream. Many Southern leaders wanted new land for slavery, while many Northerners feared exactly that. This split, barely hidden in 1846, would break open over the land the war brought in, as the Legacy tab explains.
Americans had filed claims against Mexico for years over seized ships and property and unpaid debts. An 1839 claims commission awarded Americans about $2 million, and Mexico agreed to pay in installments, but a treasury drained by revolutions and war stopped paying after 1843. The unpaid claims gave Polk a lever to use in negotiations.
In November 1845 Polk sent John Slidell, a Louisiana congressman, to Mexico City as minister. Slidell was authorized to have the United States take over the claims if Mexico accepted the Rio Grande border, and to offer up to $25 million more for New Mexico and California.
Mexico's president, José Joaquín de Herrera, had been willing to talk about Texas, but receiving an American minister with full powers would have looked like restoring normal relations and accepting annexation. Under heavy criticism, his government refused to receive Slidell. In December 1845 General Mariano Paredes overthrew Herrera, partly by accusing him of being ready to give up Texas, and the new government also refused.
Slidell reported in early 1846 that Mexico would not negotiate. Polk ordered Taylor to the Rio Grande in January 1846, and by May he had drafted a message asking Congress for war on the grounds of the unpaid claims and the rejection of Slidell alone. Events on the Rio Grande overtook that plan within days.
The War, Step by Step
From the first skirmish on the Rio Grande to the fall of Mexico City, the major events of the war in order. Open any entry for the full story.
On April 23, 1846, Mexican President Mariano Paredes declared a "defensive war" against the United States. Two days later, on April 25, Mexican cavalry under General Anastasio Torrejón crossed the Rio Grande and surrounded a patrol of about 70 to 80 U.S. dragoons under Captain Seth Thornton at Rancho de Carricitos, upriver from Matamoros. About a dozen Americans were killed, and most of the rest were captured.
Word of the "Thornton Affair" reached Washington on May 9. On May 11 Polk sent Congress a war message stating that Mexico "has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil." The claim that the ground was American soil was exactly the point in dispute.
The House approved a war bill that same day by a vote of 174 to 14, and the Senate followed on May 12 by 40 to 2. Many Whigs voted yes even though they doubted Polk's account, because the bill also funded troops already under fire. The Constitution gives the power to declare war to Congress, and critics argued that Polk had used his power as commander in chief to leave Congress little real choice.
Polk signed the bill on May 13, 1846. It declared that war existed "by the act of the Republic of Mexico," authorized up to 50,000 volunteers, and appropriated $10 million. The regular U.S. Army at the time had fewer than 9,000 soldiers. In Mexico, the national Congress formally recognized a state of war that July.
Before Congress had even voted, the first real battle was fought. Mexican artillery in Matamoros had been shelling Fort Texas since May 3, and Taylor marched most of his army to his supply base at Point Isabel on the coast. On his way back, on May 8, 1846, General Mariano Arista's army blocked his path on the open prairie at Palo Alto, north of present-day Brownsville.
About 2,300 U.S. soldiers faced roughly 3,400 to 4,000 Mexican troops, according to the National Park Service. The battle was decided by artillery. Taylor's "flying artillery," light cannon that could gallop to a new position and fire quickly, tore into the Mexican lines, while Mexican cannon fired heavy solid shot that often bounced short. Major Samuel Ringgold, who had developed the flying artillery tactics, was mortally wounded.
Arista pulled back to a dry riverbed called Resaca de la Palma. On May 9 Taylor attacked through thick brush, and U.S. dragoons captured the Mexican guns. The Mexican line broke, and the retreat turned into a rout. Many soldiers drowned trying to cross the Rio Grande, and Mexican losses over the two days ran into the hundreds.
The siege of Fort Texas ended, but its commander, Major Jacob Brown, had been killed; the post was renamed Fort Brown, and the city of Brownsville grew up around it. Taylor crossed the river and occupied Matamoros on May 18. The two victories made Taylor a national hero before most Americans had heard that war had been declared.
In June 1846 Colonel (soon General) Stephen W. Kearny led the Army of the West, about 1,700 soldiers, most of them Missouri volunteers, out of Fort Leavenworth in present-day Kansas. They followed the Santa Fe Trail more than 800 miles across the plains toward New Mexico.
New Mexico's governor, Manuel Armijo, gathered militia to defend Apache Canyon east of Santa Fe, then abandoned the position without a fight. Kearny entered Santa Fe on August 18, 1846, raised the U.S. flag over the Palace of the Governors, and promised residents they would keep their property and religion. He issued a legal code for the territory and appointed the trader Charles Bent as civil governor.
Many New Mexicans resented the occupation. On January 19, 1847, Hispano and Taos Pueblo rebels killed Governor Bent at his home in Taos, along with other officials. U.S. troops under Colonel Sterling Price marched north, and in early February they shelled the fortified church at Taos Pueblo, where the rebels had taken shelter. About 150 defenders were killed.
More than a dozen captured rebels were tried and hanged, most for murder and one for treason, even though they had been Mexican citizens only months before. The treason charge troubled some officials in Washington. U.S. control of New Mexico held for the rest of the war, and the Taos Revolt remains an important and painful part of the state's memory.
On June 14, 1846, before word of the war reached the Pacific, a group of American settlers seized the Mexican post at Sonoma north of San Francisco Bay. They imprisoned the Californio commander, Mariano Guadalupe Vallejo, and raised a homemade flag showing a grizzly bear and a star, declaring a "California Republic." Army explorer Captain John C. Frémont soon took charge of the rebels.
The Bear Flag Republic lasted less than a month. On July 7, 1846, Commodore John D. Sloat of the U.S. Navy landed at Monterey, the capital, and claimed California for the United States. His successor, Commodore Robert F. Stockton, occupied Los Angeles in August. The takeover seemed nearly bloodless.
In September 1846, Californios in Los Angeles led by José María Flores rose up and drove out the small American garrison. When Kearny arrived from New Mexico with about 100 dragoons, Californio lancers under Andrés Pico attacked him at San Pasqual, near present-day Escondido, on December 6, 1846. About 21 Americans were killed, making it the costliest fight for U.S. forces in California.
Combined forces under Stockton and Kearny won small battles at the San Gabriel River and La Mesa on January 8 and 9, 1847, and retook Los Angeles. On January 13 Frémont and Andrés Pico signed the Treaty of Cahuenga, ending the fighting in California. The Mormon Battalion, about 500 Latter-day Saint volunteers, reached San Diego at the end of that month after marching nearly 2,000 miles from Iowa.
After a summer spent waiting for volunteers and supplies, Taylor marched about 6,000 men into the interior toward Monterrey, the main city of northeastern Mexico. General Pedro de Ampudia defended it with roughly 7,000 soldiers, stone buildings, and hilltop forts, including the fortified Bishop's Palace.
The attack began on September 21, 1846. One U.S. division under General William J. Worth swung around to the west and stormed the hills, while other troops attacked from the east and suffered heavy losses. By September 23, American soldiers were fighting street by street, tunneling through the walls of houses to avoid gunfire from the flat rooftops.
On September 24 Ampudia asked for terms. Taylor agreed to let the Mexican army march out with its weapons and to an eight-week armistice. Taylor's army had lost nearly 500 killed and wounded, and he believed generous terms would help open peace talks.
Polk was furious that the Mexican army had been allowed to escape and ordered the armistice ended. The president, a Democrat, also distrusted Taylor, whose victories were making him a likely Whig presidential candidate. Taylor occupied Saltillo in November, but Polk decided the war would be won with a different general and a different plan.
In 1846 Antonio López de Santa Anna was living in exile in Havana, Cuba, after being overthrown in 1844. He sent word to Polk that if he returned to Mexico he could arrange a peace that gave the United States the land it wanted. Polk ordered the U.S. Navy to let him through its blockade of the Mexican coast.
Santa Anna landed at Veracruz on August 16, 1846. Instead of making peace, he took command of the Mexican army and began raising a new force at San Luis Potosí. Polk's gamble had put Mexico's most experienced general back in the field.
In December 1846 the Mexican Congress named Santa Anna president, with Valentín Gómez Farías as vice president running the government while Santa Anna led the army. Mexico was nearly bankrupt, and in January 1847 Gómez Farías pushed through a law to raise money by seizing and selling Church property.
The law set off the "Polkos" revolt in Mexico City in late February 1847, in which militia units made up largely of well-to-do citizens rose against the government while the U.S. was preparing to invade. The revolt ended only when Santa Anna returned to the capital in March and removed Gómez Farías, a sign of how divided Mexico's leaders were throughout the war.
When Kearny left for California, he ordered Colonel Alexander W. Doniphan and his 1st Missouri Mounted Volunteers, about 850 men, to march south into the Mexican state of Chihuahua. Before leaving, Doniphan's troops negotiated a treaty with Navajo leaders at Bear Spring in November 1846.
On Christmas Day 1846, Doniphan's men defeated a Mexican force at El Brazito, near present-day Las Cruces, and then occupied El Paso del Norte, the town that is now Ciudad Juárez. They continued south across the Chihuahuan Desert.
On February 28, 1847, they met a larger Mexican army entrenched at the Sacramento River, just north of the city of Chihuahua. Using artillery and a flanking march across a gully, the Missourians broke the Mexican position. Mexican losses were in the hundreds, while Doniphan reported only one man killed. The volunteers entered Chihuahua on March 1.
Doniphan's men then marched east to join Taylor's army and went home by way of New Orleans. By the end of their enlistment they had traveled several thousand miles by foot, horse, and boat, one of the longest marches in U.S. military history, which made Doniphan a celebrated figure in Missouri.
For his new campaign against Veracruz, General Winfield Scott took most of Taylor's regular troops, leaving Taylor in northern Mexico with about 4,700 men, most of them volunteers who had never been in battle. A captured letter told Santa Anna about the transfer, and he marched north from San Luis Potosí with about 20,000 soldiers to destroy Taylor's weakened army.
The desert march cost Santa Anna thousands of men to hunger, cold, and desertion, but he still arrived with roughly 15,000. Taylor took up a defensive position at a narrow mountain pass near the hacienda of Buena Vista, south of Saltillo. On February 22, 1847, Santa Anna invited him to surrender; Taylor refused.
The main battle on February 23 swung back and forth across the plateau. Mexican attacks broke several volunteer regiments, and at one point the American left seemed about to collapse. U.S. artillery under Captains Braxton Bragg and Thomas W. Sherman and the 1st Mississippi Rifles under Colonel Jefferson Davis helped hold the line. The United States lost more than 700 killed and wounded; Mexican losses were far higher.
That night Santa Anna withdrew, citing his army's exhaustion and the political crisis in Mexico City, and claimed a victory. For Americans, Buena Vista made Taylor even more famous. It was the last major battle in northern Mexico, and the war's outcome now turned on Scott's campaign.
Scott's plan was to capture Veracruz, Mexico's main port, and follow the old road inland to Mexico City, the route Hernán Cortés had taken in 1519. The War Department and Navy gathered troops, supplies, and dozens of specially built surfboats for the landing.
On March 9, 1847, about 10,000 U.S. soldiers landed on Collado Beach south of the city in a single day, rowed ashore in the surfboats with naval gunfire covering them. It was the largest amphibious landing in U.S. history up to that time, and not one man was lost in the landing itself.
Veracruz was a walled city defended by more than 3,000 Mexican troops and the island fortress of San Juan de Ulúa. Rather than risk a costly assault, Scott surrounded the city and, beginning on March 22, bombarded it with army guns and heavy naval cannon brought ashore. Civilians died along with soldiers, with estimates of the civilian dead running into the hundreds, and the bombardment was widely condemned in Mexico.
The city surrendered on March 27, and U.S. troops marched in on March 29. Scott was in a hurry to move inland before the season of yellow fever, which the locals called "el vómito," arrived on the hot, low coast. He set out on the National Road toward the highlands in early April.
Santa Anna chose to stop Scott at Cerro Gordo, where the National Road climbs from the coastal plain through a narrow mountain pass. He fortified the hills overlooking the road with about 12,000 men and believed the rough ground on his left could not be crossed by an army.
Scott's engineers, including a young captain named Robert E. Lee, scouted a path through the ravines around the Mexican left. On April 17 and 18, 1847, U.S. troops used that route to attack the Mexican position from the side and rear while others struck from the front. The Mexican army collapsed.
About 3,000 Mexican soldiers were captured, along with dozens of cannon, and Scott released most of the prisoners on parole because he could not feed or guard them. The United States lost more than 400 killed and wounded. Santa Anna barely escaped, leaving behind his carriage.
Soldiers of the 4th Illinois Volunteers found one of Santa Anna's artificial legs in the carriage and took it home. It is still displayed at the Illinois State Military Museum in Springfield, and Mexico has asked more than once for its return. After Cerro Gordo, Scott's army advanced to the highland city of Jalapa.
Scott occupied Puebla, Mexico's second-largest city, on May 15, 1847, without a fight. He kept his troops under strict discipline and paid for supplies, hoping to avoid the guerrilla war that had followed the occupation of northern Mexico.
The army then shrank sharply. The enlistments of seven volunteer regiments, about 3,000 to 4,000 men, ran out, and nearly all of them went home. Scott waited in Puebla for most of the summer until reinforcements brought his strength to roughly 10,000 soldiers.
Meanwhile Polk had sent Nicholas P. Trist, the chief clerk of the State Department, to travel with the army and negotiate peace whenever Mexico was ready. Scott and Trist at first quarreled bitterly through letters, then became close allies after Trist fell ill and Scott sent him supplies.
On August 7, 1847, Scott marched out of Puebla toward Mexico City. To move fast, he cut loose from his long supply line to the coast and planned to live off the country. Many observers in Europe expected the small army, deep in enemy country, to be destroyed.
Mexico City was protected by lakes, marshes, and causeways. Scott avoided the heavily defended eastern approach and swung his army around to the south. On August 19 and 20, 1847, U.S. troops attacked General Gabriel Valencia's force near the village of Padierna, a fight Americans called the Battle of Contreras. A surprise attack at dawn on August 20 routed Valencia's army in minutes.
The same day, U.S. troops attacked the fortified convent of San Mateo at Churubusco and a nearby bridgehead. The convent was defended by Generals Manuel Rincón and Pedro María Anaya, Mexican National Guard units, and the Saint Patrick's Battalion, a unit made up largely of deserters from the U.S. Army. The defenders held out for hours until their ammunition ran out.
According to a well-known Mexican tradition, when U.S. General David Twiggs asked Anaya to hand over his ammunition, Anaya replied, "Si hubiera parque, no estarían ustedes aquí" ("If we had ammunition, you would not be here"). The words are carved on monuments in Mexico, though accounts of the exact exchange differ.
The two battles cost the United States more than 1,000 killed and wounded. Mexican losses, killed, wounded, and captured, were several times higher. On August 24 both sides agreed to an armistice so that Trist could meet Mexican commissioners. The talks failed over the border and the status of New Mexico, and Scott ended the armistice on September 6.
On September 8, 1847, Scott sent General Worth to seize a group of stone buildings called Molino del Rey (the King's Mill), where he had been told the Mexicans were casting cannon. The report proved false, and the position was strongly defended. Worth took it, but his division lost nearly 800 killed and wounded, the bloodiest day of the campaign for the United States.
The last strong point before the city was Chapultepec Castle, on a steep hill that also housed Mexico's national military academy. On September 12 U.S. artillery pounded the castle all day. On the morning of September 13, divisions under Generals Gideon Pillow and John A. Quitman stormed the hill, climbing the walls with scaling ladders. General Nicolás Bravo's garrison of about 1,000 men could not hold.
Among the defenders were cadets of the military academy, some of them teenagers. Six of them, Juan de la Barrera, Juan Escutia, Francisco Márquez, Agustín Melgar, Fernando Montes de Oca, and Vicente Suárez, died in the fighting. Mexico honors them as the Niños Héroes (Boy Heroes). According to tradition, Escutia wrapped himself in the Mexican flag and leapt from the castle to keep it from being captured; historians note the story is not documented in accounts from the time.
The same morning, at nearby Mixcoac, the U.S. Army hanged 30 captured members of the Saint Patrick's Battalion, timing the executions to the moment the U.S. flag rose over Chapultepec. In all, about 50 members of the battalion were hanged in September 1847. Their story is told further in the Key Figures tab.
After Chapultepec fell, U.S. troops fought their way along the causeways to the Belén and San Cosme gates of the city. That night Santa Anna withdrew his remaining army. On the morning of September 14, 1847, Quitman's troops entered the main plaza, and the U.S. flag was raised over the National Palace. Scott rode in later that morning, the scene shown in this page's main picture.
For the first days, residents fought the occupiers from rooftops and windows, and U.S. troops answered with artillery and house searches before order was restored. Scott placed the city under martial law, collected a levy from the city government to pay for his army, and punished crimes by his own soldiers as well as by Mexicans.
Santa Anna resigned the presidency and tried to cut Scott's supply line by besieging the small American garrison left at Puebla. The siege failed, and a relief column defeated his troops at Huamantla on October 9, 1847. The Mexican government removed him from command, and he later left the country. Guerrilla bands continued to attack U.S. supply trains on the road from Veracruz.
Manuel de la Peña y Peña, president of the Supreme Court, took over as interim president and moved the government to Querétaro. The occupation of Mexico City lasted nine months, until the last U.S. troops left the capital on June 12, 1848.
After the failed armistice talks, Polk decided Trist had been too willing to bargain. On October 6, 1847, he ordered Trist home. The recall letter reached Trist in mid-November, just as a new Mexican government signaled that it was ready to negotiate seriously.
Trist, with Scott's support, concluded that if he left, the chance for peace might be lost and the war might go on indefinitely. He stayed and kept negotiating, explaining his decision to the Secretary of State in a letter of about 65 pages. Polk, reading it in Washington, was outraged.
Trist met with three Mexican commissioners, Bernardo Couto, Miguel Atristáin, and Luis Gonzaga Cuevas, through January 1848. On February 2, 1848, they signed the treaty in the village of Guadalupe Hidalgo, next to the Basilica of Our Lady of Guadalupe just north of Mexico City. The terms were close to what Polk had originally authorized.
Trist was fired from the State Department and was not paid for his time in Mexico. Congress finally paid his salary and expenses in 1871. The treaty itself, how it was approved, and what it meant for the people living in the ceded land are covered in the Treaty & Aftermath tab.
The Treaty and What Came After
The Treaty of Guadalupe Hidalgo ended the war, set a new border, and left questions about land, citizenship, and slavery that took years to settle.
The treaty's formal name was the Treaty of Peace, Friendship, Limits, and Settlement between the United States of America and the Mexican Republic. Article V set the new border: up the Rio Grande from the Gulf to the southern boundary of New Mexico, west to the Gila River, down the Gila to the Colorado River, and then in a straight line to the Pacific just south of San Diego.
Mexico recognized the Rio Grande as the border of Texas and gave up about 525,000 square miles. That land later became all of California, Nevada, and Utah, most of Arizona, and parts of New Mexico, Colorado, and Wyoming.
In return, under Article XII, the United States agreed to pay Mexico $15 million: $3 million once the treaty was ratified and the rest in yearly installments with 6 percent interest. The United States also agreed to take over up to $3.25 million in claims that American citizens held against Mexico, the old debt dispute from before the war.
Article XI made the United States responsible for stopping raids by Native nations, such as the Comanche and Apache, from its new territory into Mexico and for paying for the damage. The obligation proved impossible to meet, and Mexico filed large claims under it until the Gadsden Purchase treaty canceled it in 1854.
Polk was angry at Trist but could not reject a treaty that gave him what he had asked for. Some Democrats now wanted to take "all Mexico," while many Whigs wanted no new land at all, and the House had turned against the war. Polk sent the treaty to the Senate on February 23, 1848.
The Senate approved it on March 10, 1848, by a vote of 38 to 14, after striking Article X, which would have protected Mexican land grants, and changing Article IX on the rights of residents. American envoys signed the Protocol of Querétaro to explain the changes to Mexico, stating that valid land titles would still be recognized under U.S. law.
In Mexico, the treaty was bitterly debated by the Congress meeting at Querétaro, and some members argued for continuing the war. The Chamber of Deputies and Senate approved it in May, and the two governments exchanged ratifications at Querétaro on May 30, 1848. Polk proclaimed the treaty in effect on July 4, 1848.
U.S. troops left Mexico City on June 12, 1848, and the last American soldiers sailed from Veracruz in early August. The Mexican government returned to its capital. Under the treaty, the United States also returned captured Mexican forts and customs houses and gave back the property it had seized during the occupation.
The treaty moved the border, not the people. Estimates of the number of Mexican citizens living in the ceded territory commonly run from about 80,000 to 100,000, most of them in New Mexico, along with far larger numbers of Native people who were not counted as Mexican citizens.
Articles VIII and IX let Mexican citizens in the territory choose to stay. Those who did not declare within one year that they wished to remain Mexican would become U.S. citizens, and their property was to be "inviolably respected." A few thousand moved south to Mexico; most stayed and became Americans, one of the earliest large groups to gain U.S. citizenship by treaty rather than by birth or naturalization.
Keeping their land proved much harder. The California Land Act of 1851 required every owner of a Spanish or Mexican land grant to prove the title before a federal commission, a process that often took years and heavy legal fees. Many Californio families lost their ranchos to lawyers' fees, debts, and squatters, even when their titles were eventually confirmed.
In New Mexico, the federal Court of Private Land Claims, created in 1891, confirmed only a small share of the acreage claimed under old grants, and many shared village lands were lost. Disputes over these grants continued into the twentieth century, and the treaty is still cited today in debates over land and water rights in the Southwest.
On January 24, 1848, nine days before the treaty was signed and before anyone in Mexico City knew of it, James W. Marshall found gold at John Sutter's sawmill on the American River in California. When Polk confirmed the discovery in his December 1848 message to Congress, the Gold Rush began in earnest.
About 300,000 people came to California from across the United States and around the world in the following years. The population grew so fast that Californians wrote a state constitution in 1849 without first going through the usual stage as a territory, and that constitution banned slavery.
California's request to join the Union as a free state set off a national crisis, because it would upset the balance of 15 free and 15 slave states in the Senate. Southern leaders threatened secession, and Congress argued for most of 1850 over what to do with all the land taken from Mexico.
The Compromise of 1850 admitted California as a free state on September 9, 1850, organized New Mexico and Utah as territories that could decide the slavery question for themselves, settled a border dispute with Texas, and passed a stronger Fugitive Slave Act. The compromise held off disunion for a decade, but it did not settle the underlying conflict, as the history of Black Americans in the 1850s shows.
The treaty's border left two problems. The mapping errors on the Disturnell map left the Mesilla Valley, near present-day Las Cruces, claimed by both countries, and the best route for a southern railroad to California ran through Mexican land south of the Gila River.
In 1853 President Franklin Pierce sent James Gadsden, a South Carolina railroad promoter, as minister to Mexico. Santa Anna, back in power once more and short of money, agreed to sell. The treaty was signed in Mexico City on December 30, 1853.
The U.S. Senate cut the land to be purchased and the price, and approved a final treaty in 1854. The United States paid $10 million for about 29,670 square miles that became southern Arizona and southwestern New Mexico, including the site of Tucson. The treaty also canceled Article XI of the 1848 treaty and its duty to stop cross-border raids.
The sale was so unpopular in Mexico that it helped bring down Santa Anna's last government in 1855. The Southern Pacific Railroad later built its line through the purchased land in the early 1880s. With the Gadsden Purchase, the land border between the continental United States and Mexico took its current shape, apart from small later adjustments along the Rio Grande.
The Map in the Treaty
The negotiators drew the new border on John Disturnell's 1847 map of Mexico, which was attached to the treaty. The map had errors: it placed the town of El Paso about 34 miles north and 100 miles east of its true location. That mistake led to a dispute over the Mesilla Valley that was settled only by the Gadsden Purchase in 1853.
The War by the Numbers
| Measure | Figure | Notes |
|---|---|---|
| U.S. service members | 78,718 | Regulars and volunteers combined (Department of Veterans Affairs) |
| U.S. battle deaths | 1,733 | Killed in action or died of wounds (VA) |
| U.S. other deaths in service | 11,550 | Mostly disease, such as dysentery, measles, and yellow fever; about 6.7 times the battle deaths (VA) |
| U.S. wounded | 4,152 | Wounded who survived (VA) |
| Mexican casualties | About 25,000 (estimate) | Killed and wounded, military and civilian; Mexico kept no complete count, and estimates vary |
| Payment to Mexico | $15 million | Treaty of Guadalupe Hidalgo, Article XII |
| Claims assumed by the U.S. | Up to $3.25 million | Americans' claims against Mexico, Articles XIV and XV |
| Land ceded by Mexico | About 525,000 sq mi | The Mexican Cession; 55 percent of Mexico's territory when Texas is included (National Archives) |
| Gadsden Purchase | $10 million | About 29,670 square miles added in 1853 to 1854 |
Key Figures
The presidents, generals, diplomats, and soldiers on both sides whose decisions shaped the war.
James K. Polk of Tennessee, a protégé of Andrew Jackson, won the 1844 election on a platform of expansion. He promised to serve only one term and set four goals: lower tariffs, an independent treasury, settling the Oregon boundary, and acquiring California. He achieved all four.
Polk directed the war closely from the White House, working long days and planning strategy with his cabinet. He is often described as the first president to act as a true wartime commander in chief, choosing the generals and approving their campaigns.
He also distrusted his two leading generals, Taylor and Scott, because both were Whigs who might run for president. His private diary, published in 1910, records his frustration with them and with Trist, and it is one of the main sources historians use to judge his aims.
Supporters credit Polk with adding more territory to the United States than any president except Thomas Jefferson. Critics, then and since, argue that he maneuvered the country into war by sending troops into disputed land. Polk left office in March 1849 and died of cholera three months later, at 53.
Antonio López de Santa Anna dominated Mexican politics for three decades and served as president, by most counts, 11 separate times between 1833 and 1855. He switched between federalist and centralist sides as circumstances changed and was repeatedly overthrown and exiled.
He was already famous in the United States as the commander who defeated the Alamo's defenders and then lost at San Jacinto in 1836. In 1838 he lost part of his left leg fighting a French invasion at Veracruz, and he wore artificial legs for the rest of his life.
In the 1846 to 1848 war he raised and led armies with great energy under nearly impossible financial conditions, but he lost the key battles of Buena Vista, Cerro Gordo, and the fight for Mexico City. Many Mexicans at the time, and some historians since, have accused him of misjudgment or worse, pointing to his secret contacts with Polk before his return.
Santa Anna returned to power in 1853 and sold the Gadsden Purchase land to the United States, which added to his unpopularity. Overthrown in 1855, he spent most of the rest of his life in exile and died in Mexico City in 1876.
Zachary Taylor was a career army officer who had served for nearly 40 years, mostly on the frontier. His troops called him "Old Rough and Ready" for his plain clothes and willingness to share their hardships.
He won the war's first battles at Palo Alto and Resaca de la Palma, took Monterrey, and held off Santa Anna's much larger army at Buena Vista. His generous terms at Monterrey angered Polk, and the transfer of his regular troops to Scott left him on the defensive for the rest of the war.
His victories made him a national hero. The Whig Party, which had opposed much of the war, nominated him for president in 1848, and he won the election that November against Democrat Lewis Cass and Free Soil candidate Martin Van Buren.
Taylor, a Louisiana slaveholder, surprised many Southerners as president by supporting California's admission as a free state. He died in office on July 9, 1850, after about 16 months as president, in the middle of the congressional fight over the land his army had helped win.
Winfield Scott had been a hero of the War of 1812 and had served as commanding general of the U.S. Army since 1841. He was known for his strict attention to discipline and military detail, which earned him the nickname "Old Fuss and Feathers."
He designed and led the campaign from Veracruz to Mexico City, which included the war's great amphibious landing and a string of victories over larger forces. The Duke of Wellington, Britain's most famous soldier, is reported to have followed the campaign closely and praised Scott as the greatest living general.
Scott's strict rules for his troops' conduct in occupied Mexico, including trials of American soldiers for crimes against civilians, are seen by historians as an early model of military occupation. After the war, a court of inquiry examined disputes between Scott and several of his generals, and Polk removed him from command in early 1848.
Scott ran for president as the Whig candidate in 1852 and lost to Franklin Pierce, who had served as one of his brigadier generals in Mexico. He remained the army's top general until late 1861 and proposed the "Anaconda Plan" of blockade used at the start of the Civil War.
Nicholas Trist was a Virginian who had married Thomas Jefferson's granddaughter and had served as secretary to President Andrew Jackson. In 1847 he was the chief clerk of the State Department, the department's second-ranking officer, when Polk chose him to travel secretly with Scott's army and negotiate peace.
Trist spoke Spanish, had lived in Cuba as U.S. consul in Havana, and came to know many Mexican officials. His first attempt at a settlement during the August 1847 armistice failed.
When Polk recalled him in October 1847, Trist chose to disobey, believing that his departure would end any chance of peace for a long time. The treaty he signed on February 2, 1848, is one of the few major U.S. treaties negotiated by an envoy acting against direct orders.
Polk refused to pay Trist for his work after the recall, and Trist spent years working as a clerk for a railroad. In 1871, with help from supporters in Congress, he was finally paid his salary and expenses. He died in 1874.
Manuel de la Peña y Peña was a respected lawyer and the president of Mexico's Supreme Court. When Santa Anna resigned after the fall of Mexico City in September 1847, the constitution made Peña y Peña interim president.
He moved the government to Querétaro, north of the capital, and argued that Mexico could no longer continue the war. With the treasury empty, the army scattered, and several states talking of making separate arrangements, he concluded that a negotiated peace was the only way to keep the rest of the country together.
Peña y Peña served as president twice during the crisis, from September to November 1847 and again from January to June 1848, and his government appointed the commissioners who signed the Treaty of Guadalupe Hidalgo. He defended the treaty before the Mexican Congress, which approved it in May 1848.
In June 1848 he handed the presidency to José Joaquín de Herrera, who was elected by the Congress, and Peña y Peña returned to the Supreme Court. He died in 1850. In Mexico he is remembered as the leader who accepted a painful peace to preserve the nation.
John Riley was an Irish-born soldier in the U.S. Army who deserted near Matamoros in April 1846, shortly before the war was declared, and joined the Mexican army. He organized the Batallón de San Patricio, named for Ireland's patron saint, which fought as an artillery and later an infantry unit.
The battalion was made up largely of deserters from the U.S. Army, most of them Irish and German Catholic immigrants, along with other foreign-born men living in Mexico. Historians point to several motives: harsh treatment and anti-Catholic prejudice in the U.S. Army, sympathy for a fellow Catholic nation, and Mexican offers of land and pay. The story has a special place in Irish American history.
The San Patricios fought at Monterrey, Buena Vista, and Cerro Gordo, and made their last stand at the convent of Churubusco in August 1847, where about 85 were captured. U.S. Army courts-martial sentenced most of them to death for desertion in wartime.
About 50 were hanged in September 1847. Riley and several others who had deserted before war was declared could not legally be executed; instead they were whipped and branded on the face with the letter D for deserter. In Mexico the battalion is honored as heroes, with a memorial plaque in San Ángel in Mexico City and ceremonies every September 12.
In September 1847 Chapultepec Castle housed Mexico's Colegio Militar, its national military academy. When U.S. forces attacked, some cadets stayed to fight alongside the garrison even though they had been told to withdraw.
Six cadets died in the battle on September 13, 1847: Lieutenant Juan de la Barrera and cadets Juan Escutia, Francisco Márquez, Agustín Melgar, Fernando Montes de Oca, and Vicente Suárez. The youngest were in their early teens.
Over time they became Mexico's most important symbol of the war. In March 1947, on the war's centennial, U.S. President Harry Truman laid a wreath at their monument in Chapultepec, a gesture widely noted in both countries. A large memorial, the Altar a la Patria, was dedicated in Chapultepec Park in 1952.
Some parts of the story, especially the account of Juan Escutia leaping from the castle wrapped in the flag, do not appear in records from 1847 and developed later. Historians generally treat the core facts, the cadets' presence at the battle and their deaths, as well established. September 13 is observed in Mexico as a national day of remembrance.
Legacy
How the war divided Americans at the time, how it fed the crisis over slavery, and how the United States and Mexico remember it today.
The war divided Americans from the start. Most Democrats supported it, while many Whigs called it "Mr. Polk's War." The two political parties split largely along those lines, though Whigs kept voting to fund the troops in the field.
Abraham Lincoln, a first-term Whig in the House of Representatives, introduced his "Spot Resolutions" on December 22, 1847, asking Polk to name the exact spot where American blood had first been shed and whether it was really on American soil. On January 3, 1848, the House voted 85 to 81 for an amendment stating that the war had been "unnecessarily and unconstitutionally begun by the President."
Many abolitionists opposed the war as a scheme to add slave territory. In July 1846 the writer Henry David Thoreau spent a night in jail in Concord, Massachusetts, for refusing to pay a poll tax, in part as a protest against slavery and the war. His later essay, "Resistance to Civil Government" (1849), now known as "Civil Disobedience," influenced later movements of nonviolent protest.
Supporters saw the war differently: as a defense of Texas and of American troops, a response to Mexico's refusal to negotiate, and a chance to secure the Pacific coast. Thousands of volunteers rushed to enlist in 1846, especially in the South and West, and newspapers celebrated each victory. The debate over whether the war was just continues among historians today.
On August 8, 1846, only three months into the war, Representative David Wilmot, a Pennsylvania Democrat, proposed an amendment to a war funding bill: slavery would be banned in any territory acquired from Mexico. The Wilmot Proviso passed the House, where Northern states had more seats, but failed in the Senate.
The proviso was offered again and again over the next few years and never became law, but it reshaped politics. Votes on it split along North and South lines rather than party lines. In 1848 antislavery Democrats and Whigs formed the Free Soil Party, whose slogan was "Free Soil, Free Speech, Free Labor, and Free Men."
The fight over the new land led through the Compromise of 1850 to the Kansas-Nebraska Act of 1854 and the founding of the Republican Party. The Massachusetts writer Ralph Waldo Emerson had predicted in 1846 that "the United States will conquer Mexico, but it will be as the man swallows the arsenic, which brings him down in turn."
Ulysses S. Grant, who fought in Mexico as a young lieutenant, drew the same link in his memoirs, published in 1885: "The Southern rebellion was largely the outgrowth of the Mexican war." Many historians agree that the question of slavery in the land taken from Mexico was one of the main roads to the Civil War.
The Mexican-American War was the first foreign war fought mainly by officers trained at the U.S. Military Academy at West Point, and Scott credited them with much of his success. It became part of the professional education of the officers who would lead both sides in America's next great war.
Future Union generals who served in Mexico included Ulysses S. Grant, George B. McClellan, George G. Meade, and Joseph Hooker. Future Confederate leaders included Robert E. Lee, Thomas "Stonewall" Jackson, James Longstreet, P. G. T. Beauregard, and Jefferson Davis, who later became president of the Confederacy.
Many of these men knew each other from Mexico. Lee and Grant had met there, and at Appomattox in 1865, Grant recalled in his memoirs, the two talked about their service in the old army in Mexico before turning to the terms of surrender.
The war also taught lessons about volunteers, supply, disease, and occupation. Nearly nine in ten of the U.S. soldiers who died in the war died of disease or other non-battle causes, a pattern that repeated on a much larger scale in the Civil War.
In Mexico the war is usually called la Intervención estadounidense en México (the United States Intervention in Mexico) or la guerra de Estados Unidos contra México. It is remembered as an invasion that cost the country about half of its territory, and it is a central event in Mexican history classes.
The losses deepened a period of crisis. Mexico had about a dozen changes of president during the two years of war, its treasury was empty, and it faced revolts in Yucatán and elsewhere. In 1848 a group of Mexican writers published Apuntes para la historia de la guerra entre México y los Estados Unidos, one of the first histories of the war from the Mexican side, which blamed U.S. ambition for the war while also criticizing Mexico's internal divisions.
The Niños Héroes are the war's most visible symbol in Mexico, with streets, schools, and a subway station named for them, and a national day of remembrance on September 13. The San Patricios are honored every September 12, and the defenders of Churubusco are remembered at the old convent, which now houses Mexico's National Museum of Interventions.
Historians on both sides of the border have noted that the war shaped Mexico's view of the United States for generations, and that awareness of the lost territory is part of Mexican national identity in a way that has no real parallel in U.S. memory of the same events.
In the United States the war is often called the "forgotten war," overshadowed by the Civil War that followed 13 years later. Palo Alto Battlefield National Historical Park in Brownsville, Texas, is the only unit of the National Park System dedicated to the war, and its exhibits present both the U.S. and Mexican sides.
The war survives in some familiar phrases. The first line of the Marines' Hymn, "From the Halls of Montezuma," refers to the Marines' part in the capture of Chapultepec and Mexico City, one of the proudest traditions of the U.S. armed forces.
Grant, looking back after his presidency, wrote in his memoirs: "I was bitterly opposed to the measure, and to this day regard the war, which resulted, as one of the most unjust ever waged by a stronger against a weaker nation." Other veterans and many Americans at the time viewed the war as a justified defense of Texas and a fulfillment of the nation's growth. Historians continue to debate both views.
The war's largest legacy in the United States is the Southwest itself. Cities such as Los Angeles, San Diego, San Antonio, Santa Fe, Tucson, and Albuquerque were founded under Spain or Mexico, and families in the region trace their roots to people who became Americans in 1848.
The land that changed hands in 1848 was home to many Native nations, including the Navajo, Apache, Comanche, Ute, Tohono O'odham, and the Pueblo peoples, along with dozens of tribes in California. The treaty was made without them, and the new border cut through some of their homelands. The consequences are part of the larger story of Native American history.
Mexican law had treated the Pueblo peoples and some other Native people as citizens, but U.S. law generally did not. The Pueblos' legal status and land rights were disputed in U.S. courts for decades afterward.
In California, the Gold Rush brought devastating violence against Native people. According to estimates widely cited by historians, California's Native population fell from about 150,000 in 1846 to about 30,000 by 1870, as a result of disease, starvation, and killings, including militia campaigns paid for by the state.
The Tohono O'odham Nation's homeland was split by the 1853 Gadsden Purchase line, and its members still live on both sides of today's border. A joint U.S.-Mexican commission surveyed and marked the new boundary between 1849 and 1855, creating the line that is still, with small changes, the international border.
Sources & Further Reading
- The U.S.-Mexican War, National Park Service, Palo Alto Battlefield NHP
- Treaty of Guadalupe Hidalgo (1848), National Archives
- Treaty of Guadalupe Hidalgo, full text, Avalon Project, Yale Law School
- The Mexican War and After, Library of Congress
- A Guide to the Mexican War, Library of Congress
- The Annexation of Texas, the Mexican-American War, and the Treaty of Guadalupe-Hidalgo, 1845-1848, U.S. Department of State, Office of the Historian
- Gadsden Purchase, 1853-1854, U.S. Department of State, Office of the Historian
- America's Wars, U.S. Department of Veterans Affairs
- Mexican-American War, Encyclopaedia Britannica
- Personal Memoirs of U. S. Grant, Volume 1, Project Gutenberg
- The Spot Resolutions of Abraham Lincoln, U.S. House of Representatives, History, Art & Archives
U.S. troop and casualty figures come from the Department of Veterans Affairs "America's Wars" fact sheet. Mexico kept no complete count of its losses, so Mexican casualty figures are estimates and vary between historians. Battle dates and troop strengths follow the National Park Service at Palo Alto Battlefield National Historical Park. Treaty terms are quoted from the treaty text held by the National Archives and the Avalon Project at Yale Law School.