The Mexican-American War: Even More Territory for the USA

The sociopolitical battle over the expansion of the United States and the institution of slavery within it led to conflict and the Mexican-American war.

Nov 29, 2022By Owen Rust, MA Economics in progress w/ MPA
mexico united states map 1846
A map of the United States and northern Mexico from 1846, via the Library of Congress


In the early 1840s, a crisis was brewing in the United States: the question of slavery. As the young nation expanded westward, debates erupted over whether new territories added to the nation would be slave or free. Supporters of slavery were eager to add new territories, and one ripe territory was the Republic of Texas. Texas, a sovereign nation, had only won its independence from Mexico a few years earlier. In 1845, Congress agreed to make the Republic of Texas a state. Although this was a political win for supporters of slavery, it increased tensions between the US and Mexico. When a border dispute erupted in the following year, the US sought to capitalize on the conflict for further expansion, leading to the Mexican-American war.


1821: From New Spain to Independent Mexico

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A map of New Spain circa 1750s, via the University of North Texas


Beginning in 1520, Spain colonized the territory that would eventually become Mexico. Eventually, the Viceroyalty of New Spain would spread from modern-day Panama up through the American Southwest and California. However, after the French and Indian War (1754-63), Britain emerged as the dominant imperial power in the Western Hemisphere. In the early 1800s, Spain’s power waned further as it was taken over by French dictator Napoleon Bonaparte during the Peninsular War. When Napoleon’s brother ruled Spain, its colonies in Central and South America seized the opportunity to push for freedom.


On September 16, 1810, the formal fight for Mexico’s independence from Spain began. For over a decade, fighting raged between revolutionaries and pro-Spain royalists. In 1820, a political revolution in Spain itself finally sank the royalists’ will and ability to continue to resist the push for independence. In 1821, Mexico became an independent nation. It is important to note that Mexico’s independence day is actually September 16 (Dieciseis de Septiembre), not May 5 (Cinco de Mayo)–May 5 actually commemorates the Mexican victory over France during the Battle of Puebla in 1862.


The 1820s: American Immigration into Mexico

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A map showing the US-Mexico border in the 1820s, via the Smithsonian Institution, Washington DC


When Mexico became an independent nation, it possessed vast amounts of territory in the north. Most of this was sparsely populated, with the majority of Mexico’s population in its central and southern portions. To help settle the territory and provide a hedge against Native American attacks, the government of Mexico actually encouraged some immigration from the United States! In Texas, then a province of Mexico, Stephen F. Austin brought in hundreds of American settlers in 1821.

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However, by 1830 so much immigration from the US to Mexican Texas had occurred that Mexico forbade additional immigration. It also abolished slavery in the region in 1830, intending to stem the tide of Americans bringing enslaved people to Texas, and banned slavery nationwide in 1837. White settlers from the United States also largely ignored the two requests of immigration to Mexico: learn Spanish and convert to Catholicism. By 1830, some 20,000 American families lived in northern Mexico, mostly in Texas.


1835-36: The Texas Revolution

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A painting of the Battle of the Alamo in early 1836, via the Library of Congress


In the early 1830s, in response to the two restrictions imposed on (slave-owning) American immigrants in 1830, colonist leaders in Texas began pushing for reforms. Stephen F. Austin traveled to Mexico City in 1833 and met with Mexico’s vice president, but not president Antonio Lopez de Santa Anna. Although Austin actually succeeded in reversing the immigration ban, Mexican leaders remained suspicious of the Texans’ desires for greater self-rule. In 1835, Santa Anna decided to re-militarize Texas, alarming white settlers. This militarization prompted action in September, with Austin declaring that war was the only option to prevent oppression.


The first skirmish of the war involved settlers forcibly resisting Mexican demands to hand over a cannon, leading to the famous “Come and Take It” slogan. This Battle of Gonzales on October 1, 1835 sparked a full-scale war. After swift Texan victories over small Mexican forces in autumn 1835, Santa Anna sent large armies to Texas to crush the rebellion in 1836. On March 6, a Mexican army stormed the Alamo mission, killing all defenders. The Battle of the Alamo inflamed Texan desire for revenge–as well as American hostility toward Mexico–and the Texans regrouped. On April 21, Texans under Sam Houston surprised a larger Mexican army in the Battle of San Jacinto and captured Santa Anna. As a prisoner, Santa Anna had little choice but to accept the Treaties of Velasco, which granted Texas independence.


The 1840s: Americans in California

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A map showing the Republic of Texas (east) and Alta California (west) circa 1840, via Central New Mexico Community College


Having lost some of its territory to the new Republic of Texas in 1836, Mexico also had to contend with growing populations of American settlers in Alta California. Beginning in 1834, white settlers in California received large land grants initially intended for Native Americans. In 1841, the first organized groups of white settlers began arriving overland, aided by immigrant-friendly locales built by earlier settlers arriving at California’s port cities.


Mexico had even more trouble governing distant Alta California than it had governing Texas, and by 1845, the province had largely achieved self-rule after its appointed governor fled. Around this time, the United States had been eyeing California for potential territorial expansion.  US explorers John C. Fremont and Kit Carson organized surveying expeditions into California, though they also carried military equipment. In December 1845, anticipating war, Fremont arrived in modern-day Sacramento and raised the American flag on a peak that now bears his name.


1845: Texas Becomes a State

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A Mexican map showing its assumed borders with Texas, now part of the United States, circa 1847, via the National Archives


The United States eyed both Texas and California in the early 1840s. Texas, however, was already an independent nation and sought admission to the Union. The Republic of Texas was concerned about future aggression by Mexico, and its relatively high population of American residents created a natural bond with the United States. Initially, the US avoided pursuing annexation of Texas due to threats of Mexican hostilities, but President John Tyler actively pursued annexation beginning in 1844.


Although Tyler’s first attempt to annex Texas was rejected by the US Senate, which must ratify all treaties by a two-thirds majority, a second attempt succeeded with the help of the newly-elected (but not yet sworn into office) President James K. Polk. Polk, a protégé of earlier President Andrew Jackson, supported slavery and westward expansion–including California and Oregon. By 1845, Americans who supported Manifest Destiny now saw the chance to make it real…by taking it from Mexico. Texas became a state on December 29, 1845, following the passage of the Treaty of Annexation on April 12, an event that caused Mexico to sever diplomatic ties with the United States.


The Mexican-American War Begins

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An 1848 painting of the divided American public’s reaction to war being declared against Mexico, via smarthistory


In early 1846, Texas was now formally part of the United States. However, there was a significant dispute between the US and Mexico regarding the borders. The US, and previously the Republic of Texas, declared that Texas began at the Rio Grande River, while Mexico insisted it began at the further-east Nueces River. This Trans-Nueces region is exactly where the fighting began: on April 25, 1846, a large force of Mexican soldiers attacked and killed several US soldiers on patrol. Days later, Mexico began bombarding a US fort on the Rio Grande with artillery fire. These twin attacks were sufficient for Congress to declare war, formally opening the Mexican-American War on May 13.


Similar to the War of 1812, public support for the Mexican-American War was not unanimous. Many in the North saw it as a blatant attempt to expand slave territory, and others saw it as an engineered attempt to achieve Manifest Destiny at the expense of lives. However, a sizable majority supported the war, especially due to the Mexican attacks in April. As a growing industrial power, there was little doubt that the US could easily defend Texas, but how far could it go in seizing Mexican territory?


The Overland Campaign

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A map of the campaigns of the Mexican-American War, via the US Army


As expected, the US quickly moved to protect its borders. American armies would move south from the Rio Grande into Mexico and from Kansas into New Mexico Territory to take Santa Fe. After taking Santa Fe against little opposition, General Kearney headed west to California (map above). American forces in Texas were under the command of General Zachary Taylor and seized the city of Monterrey. At the nearby city of Buena Vista, Mexican leader Antonio Lopez de Santa Anna, the same one who had fought the Texans a decade earlier, counterattacked in February 1847. The Battle of Buena Vista was one of the largest of the war and saw 5,000 American soldiers under Zachary Taylor repel a Mexican force three times its size.


Despite fighting a defensive war and possessing greater numbers of soldiers, Mexico’s military was often in disarray. There was little unification as a tool of national defense, and soldiers were often poorly paid, poorly trained, and poorly treated by officers. Perhaps its greatest weakness was Mexico’s lack of industrialization. While the US had become industrialized in the early 1800s and could manufacture its own military equipment, Mexico relied on European imports. When war broke out in 1846, Mexico’s arms were antiquated compared to new weapons made in the United States. This allowed smaller numbers of American soldiers to have more firepower than larger numbers of Mexican soldiers.


The Invasion of Veracruz

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An image of the US invasion at Veracruz, Mexico, on March 9, 1847, via the Library of Congress


After the Battle of Puebla, it was clear that the United States enjoyed a technological advantage over its Mexican opponent. But how long would it take for Americans to proceed south to Mexico City? An overland campaign into central Mexico, where Mexican supply lines would be shorter, and its population would be greater, could be extremely costly. However, US forces under General Winfield Scott surprised the Mexicans with an amphibious (sea-to-land) invasion at Veracruz on March 9, 1847. Ten thousand American soldiers were quickly landed, placing them close to Mexico City.


Intense fighting continued, but on September 14, the US military finally marched into Mexico City after a victory in the intense Battle of Chapultepec the previous day. This was the first time US troops had marched on a foreign capital, as its prior invasions of foreign territory (mostly Canada during the Revolutionary War and War of 1812) were limited and ultimately unsuccessful. With its capital city taken, Mexico had no choice but to accept American demands. Its government fled to the nearby town of Guadalupe Hidalgo, and peace treaty negotiations by State Department chief clerk Nicholas Trist delivered favorable terms to the United States.


The Treaty of Guadalupe Hidalgo

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The Mexican copy of the Treaty of Guadalupe Hidalgo (1848), via the Center for Land Grant Studies


On February 2, 1848, the Treaty of Guadalupe Hidalgo officially ended the Mexican-American War. The treaty was highly favorable to the victor, with the United States seizing approximately 55 percent of Mexico’s total territory. This included all of the American Southwest (present-day New Mexico, Arizona, Colorado, Utah, and Nevada) and Alta California (present-day California). Manifest Destiny had been achieved, as the US now fully spanned the continent from the Atlantic to Pacific Oceans.


In exchange, Mexico received $15 million as “payment” for the acquired land. The US also agreed to cover any debts owed to American citizens by the Mexican government. The US Senate ratified the treaty on March 10 but removed the section that required recognition of Mexican land grants in the ceded territories. Mexicans in the ceded territory could choose to remain and become US citizens, while those who wished to remain citizens of Mexico were encouraged to move within one year.


The Mexican Cession & Slavery

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A map of the United States showing the Mexican Cession (1848) at the bottom left of the continent, via the US Department of the Interior


The vast amount of land ceded to the United States with the Treaty of Guadalupe Hidalgo was called the Mexican Cession. Of immediate concern was whether these new territories would be slave or free. The Compromise of 1850 admitted California to the union as a free state. The remaining territory between California and Texas, separated into Utah and New Mexico Territories, would be decided later. In exchange for California being a free state, the Compromise included the passing of the Fugitive Slave Act, which required the federal government to help capture and return all escaped slaves to their owners, even if they successfully made it to free states.


After the Compromise of 1850, the issue of slavery became an even more intense and controversial topic in American politics. Over the course of the decade, the nation moved closer to civil war as further compromises were needed to handle the issue of slavery. Americans who supported slavery tried to expand into territories that did not explicitly disallow it, such as Utah, New Mexico, Kansas, and Nebraska. This often prompted localized violence that amplified national tensions.


Long-Term Lessons from the Mexican-American War

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An image of fast-moving US dragoons that outmaneuvered Mexican foes during the Mexican-American War, via the Library of Congress


The swift American victory in the Mexican-American War highlighted the importance of modern military technology, industrialization, and naval forces. Although outnumbered, US soldiers were more effective than their opponents due to the adoption of new technology and tactics. This included fast-moving light cavalry dragoons, rifles instead of older muskets, and amphibious landings instead of longer marches over land. American soldiers also had a greater sense of national unity and cohesion than Mexican soldiers, as Mexico had only been an independent nation for 25 years when the war commenced. Ultimately, deep tensions between the US and Mexico remained for many decades, including further US military incursions into Mexico during the World War I era.


Many generals in the US Civil War gained ample battlefield and tactical experience during the Mexican-American War, including both Confederate general Robert E. Lee and Union general Ulysses S. Grant. General Winfield Scott, who surprised Mexico with his amphibious landing at Veracruz, again used naval power during the US Civil War fifteen years later to try and starve the Confederacy’s economy with a naval blockade. General Zachary Taylor became President of the United States as a result of his war heroism, winning the election of 1848 but dying less than two years into his first term.

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By Owen RustMA Economics in progress w/ MPAOwen is a high school teacher and college adjunct in West Texas. He has an MPA degree from the University of Wyoming and is close to completing a Master’s in Finance and Economics from West Texas A&M. He has taught World History, U.S. History, and freshman and sophomore English at the high school level, and Economics, Government, and Sociology at the college level as a dual-credit instructor and adjunct. His interests include Government and Politics, Economics, and Sociology.