Mexican-American War | Significance, Battles, Results, Timeline, & Facts (2024)

Mexico-United States [1846–1848]

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What was the Mexican-American War?

The Mexican-American War was a conflict between the United States and Mexico, fought from April 1846 to February 1848. Won by the Americans and damned by its contemporary critics as expansionist, it resulted in the U.S. gaining more than 500,000 square miles (1,300,000 square km) of Mexican territory extending westward from the Rio Grande to the Pacific Ocean. It stemmed from the annexation of the Republic of Texas by the U.S. in 1845 and from a dispute over whether Texas ended at the Nueces River (the Mexican claim) or the Rio Grande (the U.S. claim).

What did the Mexican-American War have to do with Manifest Destiny?

The concept of Manifest Destiny held that the United States had the providential right to expand to the Pacific Ocean. In 1845 the U.S. annexed the Republic of Texas, which had won de facto independence from Mexico in the Texas Revolution (1835–36). When U.S. diplomatic efforts to establish agreement on the Texas-Mexico border and to purchase Mexico’s California and New Mexico territories failed, expansionist U.S. Pres. James K. Polk found a rationale to justify an attempt to take that land by force when U.S. and Mexican troops skirmished north of the Rio Grande on April 25, 1846.

Manifest DestinyRead more about Manifest Destiny.

James K. PolkRead about James K. Polk, during whose presidency the United States acquired vast territories along the Pacific coast and in the Southwest.

Was there opposition to the Mexican-American War within the United States?

Democrats, especially those in the Southwest, strongly favoured the Mexican-American War. Most Whigs, however, viewed the war as conscienceless land grabbing, and the Whig-controlled House voted 85 to 81 to censure Democratic Pres. James K. Polk for having “unnecessarily and unconstitutionally” initiated the war. Polk claimed that invading Mexicans had “shed American blood on American soil,” and the congressman and future president Abraham Lincoln introduced the “Spot Resolutions” in an attempt to determine precisely where the initial conflict between U.S. and Mexican troops had occurred and whether it “was, or was not, our own soil at that time.”

Read more below:Spot Resolutions and Civil Disobedience: American opposition to the war

Henry David ThoreauLearn more about Transcendentalist author Henry David Thoreau, an opponent of the Mexican-American War.

What did the U.S. gain by winning the Mexican-American War?

Under the Treaty of Guadalupe Hidalgo, which settled the Mexican-American War, the United States gained more than 500,000 square miles (1,300,000 square km) of land, expanding U.S. territory by about one-third. Mexico ceded nearly all the territory now included in the U.S. states of New Mexico, Utah, Nevada, Arizona, California, Texas, and western Colorado for $15 million and U.S. assumption of its citizens’ claims against Mexico.

Treaty of Guadalupe HidalgoRead more about the Treaty of Guadalupe Hidalgo.

How the Border Between the United States and Mexico Was EstablishedLearn more about how the borders of the United States and Mexico were changed by the Mexican-American War.

How did the Mexican-American War increase sectionalism in the United States?

The Mexican-American War reopened the slavery-extension issue, which divided the North and South and which had been largely dormant since the Missouri Compromise. Abolitionists saw the war as an attempt by the slave states to extend slavery and enhance their power with the creation of additional slave states out of the soon-to-be-acquired Mexican lands. On August 8, 1846, Rep. David Wilmot of Pennsylvania attempted to add an amendment to a treaty appropriations bill. The Wilmot Proviso—banning slavery from any territory acquired from Mexico—was never passed, but it led to acrimonious debate and contributed greatly to the rising sectional antagonism.

Wilmot ProvisoRead more about the Wilmot Proviso, the congressional proposal that sought to prohibit the extension of slavery in the lands acquired as a result of the Mexican-American War.

Mexican-American War, war between the United States and Mexico (April 1846–February 1848) stemming from the United States’ annexation of Texas in 1845 and from a dispute over whether Texas ended at the Nueces River (Mexican claim) or the Rio Grande (U.S. claim). The war—in which U.S. forces were consistently victorious—resulted in the United States’ acquisition of more than 500,000 square miles (1,300,000 square km) of Mexican territory extending westward from the Rio Grande to the Pacific Ocean.

“American blood on American soil”: Polk and the prelude to war

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Mexico severed relations with the United States in March 1845, shortly after the U.S. annexation of Texas. In September U.S. Pres. James K. Polk sent John Slidell on a secret mission to Mexico City to negotiate the disputed Texas border, settle U.S. claims against Mexico, and purchase New Mexico and California for up to $30 million. Mexican Pres. José Joaquín Herrera, aware in advance of Slidell’s intention of dismembering the country, refused to receive him. When Polk learned of the snub, he ordered troops under Gen. Zachary Taylor to occupy the disputed area between the Nueces and the Rio Grande (January 1846).

On May 9, 1846, Polk began to prepare a war message to Congress, justifying hostilities on the grounds of Mexican refusal to pay U.S. claims and refusal to negotiate with Slidell. That evening he received word that Mexican troops had crossed the Rio Grande on April 25 and attacked Taylor’s troops, killing or injuring 16 of them. In his quickly revised war message—delivered to Congress on May 11—Polk claimed that Mexico had “invaded our territory and shed American blood on American soil.”

Spot Resolutions and Civil Disobedience: American opposition to the war

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Congress overwhelmingly approved a declaration of war on May 13, but the United States entered the war divided. Democrats, especially those in the Southwest, strongly favoured the conflict. Most Whigs viewed Polk’s motives as conscienceless land grabbing. Indeed, from the outset, Whigs in both the Senate and the House challenged the veracity of Polk’s assertion that the initial conflict between U.S. and Mexican forces had taken place in U.S. territory. Further, legislators were at odds over whether Polk had the right to unilaterally declare that a state of war existed. Principally at issue was where the encounter had actually taken place and the willingness of Americans to acknowledge the Mexican contention that the Nueces River formed the border between the two countries. Active Whig opposition not only to the legitimacy of Polk’s claim but also to the war itself continued well into the conflict. In December 1846 Polk accused his Whig doubters of treason. In January 1847 the by-then Whig-controlled House voted 85 to 81 to censure Polk for having “unnecessarily and unconstitutionally” initiated war with Mexico.

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Among the most-aggressive challenges to the legitimacy of Polk’s casus belli was that offered by future president Abraham Lincoln, then a first-term member of the House of Representatives from Illinois. In December 1847 Lincoln introduced eight “Spot Resolutions,” which placed the analysis of Polk’s claim in a carefully delineated historical context that sought to

obtain a full knowledge of all the facts which go to establish whether the particular spot of soil on which the blood of our citizens was so shed was, or was not, our own soil at that time.

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Ultimately, the House did not act on Lincoln’s resolutions, and Polk remained steadfast in his claim that the conflict was a just war.

Abolitionists saw the war as an attempt by the slave states to extend slavery and enhance their power with the creation of additional slave states out of the soon-to-be-acquired Mexican lands. One abolitionist who agreed with that interpretation was author Henry David Thoreau, who was incarcerated in July 1846 when he refused to pay six years’ worth of back poll taxes because he felt the U.S. government’s prosecution of the war with Mexico was immoral. Although he spent only a single night in jail (his aunt, against his wishes, paid the taxes, thus securing his release), Thoreau documented his opposition to the government’s actions in his famous book-length essay Civil Disobedience (1849), insisting that if an injustice of government is

of such a nature that it requires you to be the agent of injustice to another, then, I say, break the law. Let your life be a counter friction to stop the machine.