The Mexican-American War was fought between the United States and Mexico from 1846 to 1848. At the time, the U.S. was expanding westward and some Americans believed the nation had a Manifest Destiny to span the continent from the Atlantic to the Pacific Oceans. This expansion brought the U.S. into conflict with Mexico, which had won its independence from Spain in 1821 and controlled territory including modern-day California, New Mexico, Utah, Nevada and parts of Arizona, Colorado and Wyoming. When Texas won independence from Mexico in 1836 and petitioned to join the United States in 1845, border disputes and Mexican resentment over the annexation led to war in 1846.
The question of public support for the war has been debated by historians. At the outset, there was enthusiasm among Democrats who saw the war as an opportunity to expand slavery and the nation’s borders. Some Whigs opposed the war on moral grounds or out of opposition to adding more slave states to the Union. But patriotic fervor and calls to support the troops led many Americans to back the war effort. Victory in the war added 500,000 square miles to U.S. territory, but also raised divisive issues that contributed to the American Civil War.
Causes of the War
Several events led up to the start of the Mexican-American War in 1846:
– Texas won independence from Mexico in 1836 and became the Lone Star Republic. American settlers heavily populated Texas and sought to join the United States.
– In 1845, Congress voted to annex Texas as a new state. Mexico still considered Texas part of its territory.
– President James K. Polk sent U.S. troops under General Zachary Taylor to the disputed border region between Texas and Mexico.
– In April 1846, Mexican troops attacked Taylor’s forces north of the Rio Grande River, killing over a dozen Americans. Polk declared war on Mexico citing the spilled American blood.
– Some Americans saw war as an opportunity to seize California and the Southwest from Mexico. Others opposed the war on moral grounds.
– Polk’s claims that Mexico “invaded our territory and shed American blood” rallied patriotic support for the war.
Political Divisions Over the War
The Mexican-American War heightened divisions between the two major political parties of the era:
– Generally supported the war with Mexico.
– Saw the war as a chance to expand slavery and Democratic power.
– Argued Manifest Destiny gave Americans a right to settle the West.
– Key figures: President Polk, Senator Lewis Cass of Michigan, Senator John C. Calhoun of South Carolina.
– Generally opposed the war and expansion.
– Saw the war as driven by slaveholders’ lust for more territory.
– Opposed adding new slave states in the West.
– Key figures: Abraham Lincoln, Senator Thomas Corwin of Ohio.
Democratic Support for Expansion
Most Democrats enthusiastically supported expansionist goals in waging war on Mexico:
– Wanted to annex Texas, California, and other Mexican lands.
– Sought new southern slave states to expand slavery and their power base.
– Argued it was America’s Manifest Destiny to control the continent.
– Saw the war as an economic opportunity to gain Pacific ports in California.
– Used patriotic propaganda to portray the war as defending American honor.
Whig Opposition to the War
Many leaders of the Whig Party criticized the Mexican War on moral and pragmatic grounds:
– Saw the war as an unjust land grab that would expand slavery.
– Warned annexing Mexico could bring non-white people into the U.S. population.
– Opposed the high cost of the war and military spending.
– Thought the war was driven by Southern slaveholders’ greed for land.
– Key Whig figure Abraham Lincoln challenged Polk’s claims about the initial border skirmish in his “Spot Resolutions.”
Public Opinion Among Soldiers and Civilians
Soldiers and civilians held a range of views on the Mexican War:
– Most volunteered to serve out of a sense of patriotism and adventure.
– Soldiers were ambivalent about Mexicans but respected their military abilities.
– Soldiers’ views were complicated by the daily brutalities of war.
– Low pay and bad conditions reduced morale, especially near the war’s end.
– Initially supported the war out of patriotism and faith in the president.
– Some saw the war as fulfilling America’s continental destiny.
– Others opposed the war on moral and pragmatic grounds.
– Support declined as the war dragged on with no end in sight.
Troop Morale During the War
The experience of serving in the Mexican War shaped soldiers’ morale and attitudes:
– In early battles like Monterrey, troops were fired up to fight with patriotic fervor.
– The Mexican landscape and guerrilla tactics took a toll on U.S. troops used to Eastern terrain.
– Long marches, bad food and water, and bloody sieges wore down troops as the war dragged on.
– Soldiers complained of boredom during long encampments waiting for supplies or orders.
– Deserters became a problem starting in 1847 as war fatigue set in.
Divided Civilian Responses
The American public was split in their reactions to the Mexican War:
– At first, many supported the war out of patriotism, faith in Polk, and belief in Manifest Destiny.
– This early unity faded as Whigs attacked the president and the war’s high monetary and human costs became clear.
– Abolitionists and some Whigs protested the war as an effort to expand slavery.
– Catholic and working-class Mexican-Americans faced discrimination and attacks from nativists.
– Opposition and war fatigue grew throughout 1847 with no end to the conflict in sight.
Territorial Gains and Costs of the War
The Mexican Cession added an immense amount of land to the United States at a high cost:
– Treaty of Guadalupe Hidalgo ended the war in 1848.
– Mexico ceded 525,000 square miles including California and the Southwest.
– Fulfilled American dreams of controlling the continent.
– Brought 100,000 Mexican citizens into the expanded U.S.
Human and Material Costs
– 13,283 American soldiers died, mostly of disease.
– Cost the U.S. over $100 million, straining the economy.
– Mexico lost 25,000 troops killed or wounded.
– Added tensions over extending slavery to new Western lands.
Table: Territorial Changes From the Mexican Cession
|423,966 square miles
|81,864 square miles
|82,168 square miles
|72,688 square miles
|121,589 square miles
|66,595 square miles
|97,914 square miles
Lasting Impact of the War
The Mexican-American War had major consequences for both nations:
For the United States
– Won vast Western lands that reached the Pacific, fulfilling beliefs in Manifest Destiny.
– Sparked ongoing disputes over whether to expand slavery into the new territories.
– Exposed sectional divides that led to the Civil War after 1850.
– Boosted support for anti-slavery Free Soil movement in the North.
– Lost half its territorial extent in 1848.
– Cession fueled resentment toward the U.S. for decades to come.
– Weakened Mexico’s economy and political stability.
– Renewed conflicts with Native American tribes displaced by U.S. expansion.
The Mexican-American War increased the size of the United States by a third through conquest. But the nation paid a high moral and political price that contributed to the Civil War after 1850. The mixed motives behind the war and its expansion of slavery raised questions about whether the United States’ growth was benign or unjust.
The Mexican-American War stirred heated national debates over America’s role in the world. Was it the nation’s manifest destiny and right to expand across the continent? Or an aggressive, unjustified war of conquest to extend slavery? Soldiers fought bravely for a mixture of patriotic and personal reasons. But as the conflict dragged on with growing costs, many Americans turned against the war. Its territorial gains were vast, adding enormous lands that finally fulfilled dreams of a United States “from sea to shining sea.” However, it also planted the seeds of disputes over slavery that ripped the nation apart in civil war after 1850. The mingled motives behind America’s first foreign war of expansion raised profound questions about the nation’s character and global ambitions.