The political and social struggle over U.S. territorial expansion and the issue of slavery led to the conflict and the Mexican-American War.
In the early 1840s, the United States faced an increasing crisis: the question of slavery. As this young nation continued to expand westward, debates erupted over whether the new territories being annexed would allow slavery. Those in favor of slavery were eager to expand the territory, and one of the most potential regions at that time was the Republic of Texas. Texas, an independent nation, had only recently gained independence from Mexico a few years earlier.
In 1845, the U.S. Congress decided to make the Republic of Texas a state. While this was a significant political victory for pro-slavery advocates, it also heightened tensions between the U.S. and Mexico. When border disputes erupted the following year, the U.S. quickly capitalized on the conflict to further its territorial ambitions, officially starting the Mexican-American War.
1821: From New Spain to Independent Mexico
Starting in 1520, Spain began colonizing the territory that would later become Mexico. Over time, the Viceroyalty of New Spain expanded its territory from present-day Panama to the southwestern U.S. and California. However, after the French and Indian War (1754-63), Great Britain emerged as the dominant imperial power in the Western Hemisphere. By the early 19th century, Spain’s power continued to decline under Napoleon Bonaparte’s occupation during the Peninsular War. When Napoleon’s brother became the king of Spain, the colonies in Central and South America seized the opportunity to fight for independence.
On September 16, 1810, Mexico’s war for independence from Spain began. For over a decade, fierce fighting ensued between revolutionaries and Spanish loyalists. By 1820, a political revolution in Spain weakened the royalists’ resolve, and in 1821, Mexico officially became an independent nation.
It’s important to note that Mexico’s Independence Day is September 16 (Dieciséis de Septiembre), not May 5 (Cinco de Mayo), which commemorates Mexico’s victory over France in the Battle of Puebla in 1862.
The 1820s: Americans Migrate to Mexico
When Mexico became independent, its territory stretched far to the north. Much of this area was sparsely populated, while the bulk of the population resided in the central and southern regions. To encourage settlement and defend against Native American attacks, the Mexican government welcomed a limited number of American immigrants. In Texas, a province of Mexico at the time, Stephen F. Austin brought hundreds of American settlers in 1821.
However, by 1830, the number of Americans moving into Mexican Texas had become overwhelming, leading the Mexican government to prohibit further immigration. At the same time, Mexico abolished slavery in the region in 1830 to prevent the arrival of enslaved people brought by American settlers, and eventually outlawed slavery nationwide in 1837. Despite this, many white settlers from the U.S. ignored two key requirements from the Mexican government: learning Spanish and converting to Catholicism. By 1830, approximately 20,000 American families lived in northern Mexico, mostly concentrated in Texas.
The Texas Revolution (1835-36)
In the early 1830s, in response to Mexico’s two restrictions on American settlers (mainly the ownership of slaves), leaders in Texas began pushing for reforms. Stephen F. Austin traveled to Mexico City in 1833 to meet with the vice president of Mexico, but was unable to meet President Antonio López de Santa Anna. While Austin succeeded in lifting the immigration ban, Mexican leaders remained suspicious of the Texans’ greater desire for autonomy. In 1835, Santa Anna decided to militarize Texas, frightening the settlers. This decision led to action in September, when Austin declared war as the only means to prevent oppression.
The first conflict occurred when settlers refused to hand over a cannon to Mexico, leading to the famous “Come and Take It” slogan. The Battle of Gonzales on October 1, 1835, ignited a full-scale war. After several swift victories by the Texian forces over small Mexican units in the fall of 1835, Santa Anna sent large armies to Texas to suppress the rebellion in 1836. On March 6, a Mexican army stormed the Alamo, killing the entire Texian garrison. The Battle of the Alamo fueled the desire for revenge among the Texans – and Americans – who reformed their army.
On April 21, Texian forces under General Sam Houston launched a surprise attack on a larger Mexican force at the Battle of San Jacinto, capturing Santa Anna. As a prisoner, Santa Anna had no choice but to sign the Treaty of Velasco, granting Texas independence.
The 1840s: Americans in California
After losing territory to the newly independent Republic of Texas in 1836, Mexico faced increasing American settlement in Alta California. Starting in 1834, white settlers in California were granted large land subsidies originally intended for Native Americans. By 1841, organized groups of white settlers began arriving by land, supported by immigrant-friendly communities established by previous settlers at California’s seaports.
Mexico struggled to manage the remote Alta California region more than Texas, and by 1845, the area had nearly achieved self-rule after the Mexican governor fled. At this point, the United States began eyeing California as a potential territory for expansion. American explorers John C. Fremont and Kit Carson organized surveys of California, but they also brought military equipment. In December 1845, anticipating war, Fremont arrived in present-day Sacramento and raised the American flag on a mountain that would later bear his name.
1845: Texas Becomes a U.S. State
In the early 1840s, the United States eyed both Texas and California. However, Texas was an independent nation at the time, seeking to join the Union. The Republic of Texas feared potential attacks from Mexico, and its large population of Americans made it a natural connection to the U.S. Initially, the U.S. avoided annexing Texas due to concerns over provoking war with Mexico, but President John Tyler vigorously pursued the annexation from 1844.
While Tyler’s first attempt to annex Texas was rejected by the U.S. Senate (because treaties required a two-thirds majority), the second attempt succeeded with support from newly elected President James K. Polk, who had not yet taken office. Polk, a successor to President Andrew Jackson’s policies, supported slavery and western expansion, including California and Oregon. By 1845, pro-“Manifest Destiny” Americans saw the opportunity to turn this vision into reality by seizing land from Mexico.
Texas officially became a U.S. state on December 29, 1845, after the Annexation Treaty was approved on April 12 of the same year. This event prompted Mexico to sever diplomatic relations with the U.S., paving the way for military conflict.
Mexican-American War Erupts
In early 1846, Texas officially became part of the U.S. However, a significant border dispute remained between the U.S. and Mexico. The U.S. and, previously, the Republic of Texas claimed the Texas border began at the Rio Grande River, while Mexico asserted that the boundary was at the Nueces River, further east. The region between the two rivers – called the Trans-Nueces area – became the battleground. On April 25, 1846, a large force of Mexican soldiers attacked and killed several U.S. soldiers patrolling the area. A few days later, Mexico began shelling a U.S. fort on the Rio Grande. These two attacks were enough for the U.S. Congress to declare war, officially beginning the Mexican-American War on May 13, 1846.
Similar to the War of 1812, public support for the Mexican-American War was not unanimous. Many in the North viewed it as an overt attempt to expand slavery, while others saw it as a plan to achieve “Manifest Destiny” at the cost of human lives. However, most Americans, especially after Mexico’s attacks in April, supported the war. As an industrial power on the rise, the U.S. could easily defend Texas, but the question remained: How far would the U.S. go in taking Mexican land?
Overland Campaign
As expected, the U.S. quickly deployed forces to secure its border. American armies advanced south from the Rio Grande into Mexican territory and from Kansas into New Mexico Territory to seize Santa Fe. After capturing Santa Fe with little resistance, General Kearney moved west to California. Meanwhile, U.S. forces in Texas under General Zachary Taylor took the city of Monterrey. Near the town of Buena Vista, Mexico’s leader, Antonio López de Santa Anna – who had once fought the Texans a decade earlier – counterattacked in February 1847. The Battle of Buena Vista was one of the largest engagements of the war, with 5,000 U.S. troops under Taylor’s command defeating a Mexican force three times its size.
Despite fighting a defensive war and having more troops, Mexico’s military often fell into disarray. The lack of unity in organizing national defense made Mexico’s forces ineffective, while soldiers were often poorly paid, inadequately trained, and mistreated by officers. Mexico’s biggest weakness may have been its lack of industrialization. While the U.S. had industrialized by the early 1800s and could produce military equipment, Mexico had to rely on imports from Europe. When the war broke out in 1846, Mexico’s weapons were outdated compared to the modern arms produced by the U.S. This allowed smaller U.S. forces to outgun Mexico’s much larger armies.
The Veracruz Invasion
After the Battle of Puebla, it was clear that the U.S. had a technological advantage over Mexico. However, the question remained: How soon could the U.S. military advance deep into Mexico City? A land campaign in central Mexico, where Mexico’s supply lines were shorter and its population larger, could result in huge casualties. However, under General Winfield Scott’s leadership, the U.S. surprised Mexico with an amphibious invasion (from sea to land) at Veracruz on March 9, 1847. Ten thousand American troops quickly landed, placing them in a position near Mexico City’s capital.
The fierce battles continued, but on September 14, 1847, the US military finally marched into Mexico City after a victory in the fierce battle at Chapultepec a day earlier. This was the first time that the US military had marched into the capital of a foreign country, as previous invasions (mainly into Canada during the Revolutionary War and the War of 1812) were limited and unsuccessful. With the capital occupied, Mexico had no choice but to accept US demands. The Mexican government was forced to flee to the nearby town of Guadalupe Hidalgo, and peace negotiations led by the chief secretary of the US State Department, Nicholas Trist, brought terms favorable to the United States.
The Treaty of Guadalupe Hidalgo
On February 2, 1848, the Treaty of Guadalupe Hidalgo officially ended the Mexican-American War. This treaty greatly benefited the victorious side, as the United States gained control of approximately 55% of Mexico’s total territory. This territory included the entire southwestern United States (now the states of New Mexico, Arizona, Colorado, Utah, and Nevada) and Alta California (now the state of California). With the completion of the “Manifest Destiny” goal, the United States was now stretched entirely from the Atlantic to the Pacific.
In return, Mexico received $15 million in “payment” for the land it ceded. The United States also agreed to pay the debts owed by the Mexican government to American citizens. The U.S. Senate ratified the treaty on March 10, but removed the provision requiring recognition of the land ownership rights of Mexicans in the ceded territories. Mexicans living in these territories could choose to stay and become U.S. citizens, while those wishing to retain Mexican nationality were encouraged to move south within a year.
The Issue of the Mexican Cession and Slavery
The vast territory that Mexico ceded to the United States through the Treaty of Guadalupe Hidalgo is known as the Mexican Cession. Immediately afterward, the question of whether these new territories would permit slavery became a major concern. The Compromise of 1850 admitted California as a free state. The remaining territories between California and Texas, divided into Utah Territory and New Mexico Territory, would later decide the issue of slavery.
In exchange for California becoming a free state, the Compromise included the Fugitive Slave Act, which required the federal government to assist in capturing and returning runaway slaves to their owners, even if they had escaped to free states.
After the Compromise of 1850, the issue of slavery became more controversial than ever in U.S. politics. Over the next decade, the nation moved increasingly closer to civil war, as more compromises were needed to resolve the slavery issue. Pro-slavery advocates attempted to expand into territories that did not explicitly prohibit slavery, such as Utah, New Mexico, Kansas, and Nebraska. These efforts often led to local conflicts, increasing tensions across the country.
The Long-Term Lessons from the Mexican-American War
The United States’ swift victory in the Mexican-American War highlighted the importance of modern military technology, industrialization, and naval power. Despite having fewer soldiers, U.S. troops performed far more effectively than their opponents due to the adoption of new technologies and tactics. These innovations included the use of light cavalry that could move quickly, the replacement of old muskets with rifles, and amphibious landings rather than long overland marches. Additionally, American soldiers had a higher sense of national unity and patriotism compared to Mexican troops, as Mexico had only gained independence 25 years before the war broke out. However, deep tensions between the United States and Mexico persisted for many decades, including U.S. military incursions into Mexico during World War I.
Many commanders in the U.S. Civil War gained battlefield experience and tactical knowledge from the Mexican-American War, including Confederate General Robert E. Lee and Union General Ulysses S. Grant. General Winfield Scott, who had surprised Mexico with a seaborne landing at Veracruz, continued to use naval power in the U.S. Civil War 15 years later to blockade the Confederate economy. General Zachary Taylor, famed for his bravery in the war, became the U.S. President after winning the 1848 election, though he died less than two years into his presidency.
Conclusion
The Mexican-American War not only changed the political map of North America but also marked a turning point in the history of territorial expansion and the development of the United States. From territorial disputes, the conflict demonstrated the industrial power, modern military technology, and strategy of the United States against a young Mexico. Moreover, the war deepened internal contradictions within U.S. politics, especially regarding the issue of slavery, bringing the nation closer to civil war.
We hope that through this article, Thefactsofwar has provided readers with a comprehensive view of the Mexican-American War — from its causes and events to its long-term consequences. The lessons from this history remind us not only of the challenges and conflicts of the territorial expansion era but also serve as a warning about the value of peace and unity within a nation. We hope that this historical knowledge will help you better understand the events that shaped North America and the world.
Translated by: Minh Tuan
Source: thecollector.com – The Mexican-American War: Even More Territory for the USA
