The Mexican-American War, 1846–1848

Learning Objectives

  • Identify the causes of the Mexican-American War
  • Describe the outcomes of the Mexican-American War, especially the Mexican Cession of 1848

Annexation and Border Disputes

Mexico denounced the annexation of Texas as “an act of aggression, the most unjust which can be found recorded in the annals of modern history.”[1] Beyond the anger produced by the annexation, the two nations both laid claim over a narrow strip of land between the Nueces River, where Mexico drew the southwestern border of Texas, and the Rio Grande, roughly 150 miles farther west, where Texans claimed that the border lay. Neither claim was realistic since the sparsely populated area, known as the Nueces Strip, was in fact controlled by Native Americans.

A map titled “Texas Claims” indicates the borders of Mexico, Texas, the United States, and “Disputed Territory,” as well as the Rio Grande, the Arkansas River, and the Nueces River.

Figure 1. In 1845, when Texas joined the United States, Mexico insisted the United States had a right only to the territory northeast of the Nueces River. The United States argued in turn that it should have title to all land between the Nueces and the Rio Grande.

In November 1845, President Polk secretly dispatched John Slidell to Mexico City to purchase the Nueces strip along with large sections of New Mexico and California. The mission was an empty gesture, designed largely to pacify those in Washington who insisted on diplomacy before war. Predictably, officials in Mexico City refused to receive Slidell. In preparation for the assumed failure of the negotiations, Polk preemptively sent a four-thousand-man army under General Zachary Taylor to Corpus Christi, Texas, just northeast of the Nueces River. Upon word of Slidell’s rebuff in January 1846, Polk ordered Taylor to cross into the disputed territory. The president hoped that this show of force would push Mexico to sell California in order to avoid a war. Unfortunately, he badly misread the situation. After losing Texas, the Mexican public strongly opposed surrendering any more ground to the United States, which left the shaky government in Mexico City little room to negotiate. On April 24, Mexican cavalrymen attacked a detachment of Taylor’s troops in the disputed territory just north of the Rio Grande, killing eleven U.S. soldiers.

A Declaration of War

It took two weeks for the news to reach Washington. Polk sent a message to Congress on May 11th that summed up the assumptions and intentions of the United States with regard to Mexico.

Instead of this, however, we have been exerting our best efforts to propitiate her good will. Upon the pretext that Texas, a nation as independent as herself, thought proper to unite its destinies with our own, she has affected to believe that we have severed her rightful territory, and in official proclamations and manifestoes has repeatedly threatened to make war upon us for the purpose of reconquering Texas. In the meantime we have tried every effort at reconciliation. The cup of forbearance had been exhausted even before the recent information from the frontier of the Del Norte. 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.[2]

Polk knew that since hostilities already existed, political dissent would be dangerous—a vote against war became a vote against supporting American soldiers under fire. Congress passed a declaration of war on May 13th. Only a few members of both parties, notably John Quincy Adams and John C. Calhoun, opposed the measure. Upon declaring war in 1846, Congress issued a call for fifty thousand volunteer soldiers. Spurred by promises of adventure and conquest, thousands of eager men flocked to assembly points across the country. However, opposition to “Mr. Polk’s War” grew steadily as more Americans questioned the reasons behind the conflict.

A lithograph shows several members of the clergy fleeing the Mexican town of Matamoros on horseback. Each man has a young woman behind him; the horse in the foreground also carries a basket laden with bottles of alcohol. The caption reads “The Mexican Rulers. Migrating from Matamoros with their Treasures.”

Figure 2. Anti-Catholic sentiment played an important role in the Mexican-American War. The American public widely regarded Roman Catholics as cowardly and vice-ridden, like the clergy in this ca. 1846 lithograph who are shown fleeing the Mexican town of Matamoros accompanied by pretty women and baskets full of alcohol.

The small but vocal anti-slavery faction decried the decision to go to war, arguing that Polk had deliberately provoked hostilities so the United States could annex more slave territory. Illinois representative Abraham Lincoln and other members of Congress issued the Spot Resolutions in which they demanded to know the precise spot on U.S. soil where American blood had been spilled. Many Whigs also denounced the war. Democrats, however, supported Polk’s decision, and volunteers for the army came forward in droves from every part of the country except New England, the seat of abolitionist activity. Enthusiasm for the war was aided by the widely held belief that Mexico was a weak, impoverished country and that the Mexican people, perceived as ignorant, lazy, and controlled by a corrupt Roman Catholic clergy, would be easy to defeat.

The U.S. Goes to War

Mexico and Texas

U.S. military strategists had three main objectives in the Mexican-American War: to take control of northern Mexico, including New Mexico, to seize California, and to capture Mexico City. General Zachary Taylor and his Army of the Center were assigned to accomplish the first goal, and they engaged the Mexican Army in three battles in central Texas and northern Mexico: Palo Alto, Resaca de la Palma, and Monterrey (not to be confused with Monterrey, California). Taylor was victorious in all three battles but treated the Mexican troops with compassion and respect, even helping to perform last rites for some of the Mexican soldiers who died, which earned him widespread praise. For his bravery and his compassion, Taylor quickly became a hero in the eyes of the American people, and Polk appointed him commander of all U.S. forces.

Leading the Army of the South into Mexico was General Winfield Scott. Both Taylor and Scott were potential competitors for the presidency, and believing—correctly—that whoever seized Mexico City would become a hero, Polk assigned Scott the campaign to avoid elevating the more popular Taylor, who was affectionately known as “Old Rough and Ready.” Scott captured Veracruz in March 1847, and moving in a northwesterly direction from there (much as Spanish conquistador Hernán Cortés had done in 1519), he slowly closed in on Mexico City. Every step of the way was a hard-fought victory, however, and Mexican soldiers and civilians both fought bravely to save their land from the American invaders.

New Mexico and California

General Stephen Watts Kearny, commander of the Army of the West, accepted the surrender of Santa Fe, New Mexico, and moved on to California, leaving Colonel Sterling Price in command. Despite Kearny’s assurances that New Mexicans need not fear for their lives or their property, the region’s residents rose in revolt in January 1847 in an effort to drive the Americans away. Although Price managed to put an end to the rebellion, tensions remained high.

Kearny, meanwhile, arrived in California to find it already in American hands through the joint efforts of California settlers, U.S. naval commander John D. Sloat, and John C. Fremont, a former army captain and son-in-law of Missouri senator Thomas Benton. Sloat, at anchor off the coast of Mazatlan, learned that war had begun and quickly set sail for California. He seized the town of Monterey in July 1846, less than a month after a group of American settlers led by William B. Ide had taken control of Sonoma and declared California a republic (which lasted for all of 25 days). A week after the fall of Monterey, the U.S. navy took San Francisco with no resistance. Although some Californios staged a short-lived rebellion in September 1846, many others submitted to the U.S. takeover. Thus, Kearny had little to do other than take command of California as its governor.

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The War’s End

Mexico City’s defenders, including young military cadets, fought to the end. According to legend, cadet Juan Escutia’s last act was to save the Mexican flag, and he leapt from the city’s walls with it wrapped around his body. On September 14, 1847, General Scott entered Mexico City’s central plaza, the Zócalo, officially marking the capture of the capital city. While some expansionists called for the U.S. to conquer all of Mexico, the United States negotiated for peace and in 1848, both nations signed the Treaty of Guadalupe Hidalgo, which resulted in the Mexican Cession.

Mexico Under Santa Anna

So what had been happening in Mexico during the 1800s and prior to the war with the U.S.? Much of it centered around one charismatic leader: Antonio López de Santa Anna.

Mexico Gains Independence from Spain

During the Mexican War of Independence, Antonio López de Santa Anna initially fought for the Spanish Crown but joined the insurgents fighting for Mexican independence in 1821 when the Spanish King Ferdinand VII was deposed and replaced by a liberal majority with whom Santa Anna disagreed. When Mexico won independence, it was declared the Mexican Empire, but in 1824, rebels overthrew the Empire and Mexico became a Republic. Santa Anna threw his support behind the idea of Federalism, and in 1829 he became a national hero when he repelled a Spanish invasion force at Tampico, calling himself the “Napoleon of the West.”

Texas Revolution 

In 1833, Santa Anna was elected President of Mexico. Starting in 1834, he attempted a conservative, Catholic, centralist reform movement which involved disarming the civil militia, abolishing a law that exiled conservative politicians, allying with the Catholic Church, and abolishing Congress. Santa Anna effectively declared himself a dictator. These reforms sparked rebellion in several Mexican states, including Coahuila y Texas. In 1836, Santa Anna targeted the Anglo settlers living in Texas by banning any further immigration from the U.S., causing Texas to declare independence from Mexico.

While Santa Anna managed to defeat the Texian forces at the Alamo, he was defeated and captured at the subsequent Battle of San Jacinto and forced to sign the Treaties of Velasco, recognizing Texan independence from Mexico. The Mexican government denounced Santa Anna’s actions, removed him as president, and refused to recognize the Republic of Texas.

The Pastry War or the First French Intervention in Mexico

Santa Anna was allowed to return to Mexico in 1837, where he found redemption by defending the Mexican port of Veracruz from a French invasion. The French had demanded that Mexico repay the losses that French citizens had suffered during the Texas Revolution and when Mexico refused, France sent a fleet to bombard Veracruz. Santa Anna was placed in charge of the army and, although Mexico conceded some losses to France to France, Santa Anna lost most of his leg in the battle and was once again considered a hero in Mexico. This incident, the first and lesser of Mexico’s two 19th-century wars with France, preceded the French invasion of 1861–1867 which supported the short reign of Emperor Maximilian I of Mexico, whom the Mexicans executed by firing squad at the end of that later conflict.


Santa Anna was re-elected as president of Mexico in 1839 and almost immediately faced rebellion against his authority. In response, Santa Anna adopted an extreme dictatorial style of government, banning political dissent and suppressing newspapers that were critical of him. He also launched a failed military expedition into Texas, which pushed the independent Republic further toward annexation by the U.S., and attempted to raise taxes. Santa Anna’s attempts at autocracy sent his public approval plummeting again. Several Mexican states declared themselves independent of the central government and in 1844, Santa Anna relinquished his position and fled. He was eventually captured and exiled to Cuba.

Mexican-American War

However, Santa Anna could not stay gone long and in 1846, he tricked U.S. President James Polk into helping him get back to Mexico by promising that he would cooperate with the U.S. attempts to gain territory in Northern Mexico. However, Santa Anna went back on his promise to Polk and was placed at the head of the Mexican army once again to defend Mexico City against General Scott.

When his defense failed, Santa Anna once again went into exile, this time in Jamaica, but was recalled in 1853 by the conservative Mexican government. Having apparently not learned anything, Santa Anna styled himself “His Serene Highness, Hero [benemérito] of the nation, General of Division, Grand Master of the National and Distinguished Order of Guadalupe, Grand Cross of the Royal and Distinguished Spanish Order of Carlos III, and President of the Mexican Republic,” and dictator-for-life, although in reality, his position was quite precarious.

When he botched the Gadsden Purchase (the sale of land in southern Arizona and New Mexico to the U.S.), the liberal faction of the Mexican government had had enough and forced Santa Anna to resign, sending him into exile for the final time in 1855. Santa Anna traveled from Cuba to Europe, to the U.S. until 1874, when he was offered amnesty by the Mexican president and returned to Mexico City, where he died in 1876 and was buried with full military honors. Santa Anna’s influence on Mexico in the 19th century was so great that some historians call it the “Age of Santa Anna.”

The Mexican Cession

The Treaty of Guadalupe Hidalgo, signed in February 1848, was a triumph for American expansionism under which Mexico ceded nearly half its land to the United States. The Mexican Cession, as the land west of the Rio Grande was called, included the current states of California, New Mexico, Arizona, Nevada, Utah, and portions of Colorado and Wyoming. Mexico also recognized the Rio Grande as the border with the United States. Mexican citizens in the ceded territory were promised U.S. citizenship in the future when the territories they were living in became states, or they could return to Mexico within a year. In exchange, the United States agreed to assume $3.35 million worth of Mexican debts owed to U.S. citizens, paid Mexico $15 million for the loss of its land, and promised to guard the residents of the Mexican Cession against Native American raids.

As extensive as the Mexican Cession was, some argued the United States should not be satisfied yet. The “All Mexico” movement, spearheaded by many eastern Democrats, proposed that the only way to keep peace in the region was to annex all of Mexico to the U.S. Some saw it as an anti-slavery move since the population of Mexico was generally opposed to slavery. However, Southerners who opposed the “All Mexico” movement, such as South Carolina senator John C. Calhoun, did not want to make Mexico’s large mestizo (people of mixed Native and European ancestry), Catholic, non-English-speaking, revolution-prone population part of the United States. They felt that a group so diametrically opposed to White, Protestant, Anglo-American culture would cause too much tension and conflict and might also threaten Anglo racial purity.

The U.S.-Mexican War had an enormous impact on both countries. The American victory helped set the United States on the path to becoming a world power. It elevated Zachary Taylor to the presidency and served as a training ground for many of the Civil War’s future commanders. Most significantly, however, Mexico lost roughly half of its territory. Yet the United States’ victory was not without danger. Ralph Waldo Emerson, an outspoken critic, predicted ominously at the beginning of the conflict, “We will conquer Mexico, but it will be as the man who swallows the arsenic which will bring him down in turn. Mexico will poison us.”[3] Indeed, the conflict over whether to extend slavery into the newly won territory pushed the United States ever closer to disunion and civil war.

Mexican Cession 1848. The area covers modern-day California, Nevada, Utah, and parts of Wyoming, Colorado, New Mexico, and Arizona.

Figure 4. Questions about the balance of free and slave states in the Union became even more fierce after the U.S. acquired these territories from Mexico in the 1848 Treaty of Guadalupe Hidalgo. Map of the Mexican Cession.

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Mexican Cession: the lands west of the Rio Grande ceded to the United States by Mexico in 1848, including California, Arizona, New Mexico, Nevada, Utah, and parts of Wyoming and Colorado. In exchange, the U.S. paid Mexico $15 million and assumed all Mexican debts owed to U.S. citizens. Mexican citizens in the ceded territory had one year to decide if they wanted to become U.S. citizens or go back to Mexico.

Nueces Strip: an area of about 150 miles in South Texas between the Rio Grande and Nueces Rivers, which became a disputed border between Mexico and the Republic of Texas. After the Mexican-American War it was granted to the U.S.

The Spot Resolutions: in 1846, President Polk claimed that American blood had been spilled on American soil when he was trying to persuade Congress to declare war on Mexico. He was referring to the Thornton Affair, where a Mexican force defeated an American one in the disputed Nueces Strip. While Mexico claimed that the Americans had crossed into Mexico, Polk claimed that the Americans had been attacked inside Texas. Abraham Lincoln presented the Spot Resolutions in an attempt to find out exactly which “spot” the Americans had been killed on and which country it belonged to.

Treaty of Guadalupe Hidalgo: The 1848 treaty that ended the Mexican-American War and resulted in the Mexican Cession. It also placed the border between the U.S. and Mexico at the Rio Grande River.

  1. Quoted in The Annual Register, or, a View of the History and Politics of the Year 1846, Volume 88 (Washington, DC: Rivington, 1847), 377.
  2. James K. Polk, “President Polk’s Mexican War Message,” quoted in The Statesmen’s Manual: The Addresses and Messages of the Presidents of the United States, Inaugural, Annual, and Special, from 1789 to 1846: With a Memoir of Each of the Presidents and a History of Their Administrations; Also the Constitution of the United States, and a Selection of Important Documents and Statistical Information, Vol. 2 (New York: Walker, 1847), 1489.
  3. Ralph Waldo Emerson, quoted in James McPherson, Battle Cry of Freedom: The Civil War Era (New York: Oxford University Press, 1988), 51