May marks two key anniversaries in the conflict between the United States and Mexico that set in motion the Civil War—and led to California, Texas, and eight other states joining the Union.
On May 13, 1846, the United States Congress declared war on Mexico after a request from President James K. Polk. Then, on May 26, 1848, both sides ratified the peace treaty that ended the conflict.
The conflict centered on the independent Republic of Texas, which opted to join the United States after establishing its independence from Mexico a decade earlier.
The new U.S. president, James K. Polk, also wanted Texas as part of the United States, and his predecessor, John Tyler, had a late change of heart and started the admission process before he left office. Polk and others saw the acquisition of Texas, California, Oregon, and other territories as part of the nation’s Manifest Destiny to spread democracy over the continent.
The U.S. also tried to buy Texas and what was called “Mexican California” from Mexico, which was seen as an insult by Mexico, before war broke out.
Mexico considered the annexation of Texas as an act of war. After a series of border skirmishes, President Polk asked Congress for the war declaration because, under Article I, Section 8 of the Constitution, only Congress could declare war.
In the fighting that followed, the mostly-volunteer United States military secured control of Mexico after a series of battles, and the Treaty of Guadalupe Hidalgo was signed on February 2, 1848.
It was the first large-scale success of a United States military force on foreign soil.
Mexico received a little more than $18 million in compensation from the United States as part of the treaty.
The pact set a border between Texas and Mexico and ceded California, Nevada, Utah, New Mexico, most of Arizona and Colorado, and parts of Oklahoma, Kansas, and Wyoming to the United States. Their transfer to the United States’ control also cut the territorial size of Mexico in half.
On the surface, the war’s outcome seemed like a bonanza for the United States. But the acquisition of so much territory with the issue of slavery unresolved lit the fuse that eventually set off the Civil War in 1861. But the underlying issue was how adding new states and territories would alter the balance between free and slave states was critical.
On the battlefield, Robert E. Lee, Ulysses S. Grant, and Stonewall Jackson were among those who served in the war against Mexico who would later gain prominence in the American Civil War.
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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).
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 Hidalgo
Read more about the Treaty of Guadalupe Hidalgo.Discover how the Mexican-American War ultimately helped push the U.S. closer to civil war
Learn how the Mexican-American War ultimately pushed the United States closer to civil war.
Encyclopædia Britannica, Inc.See all videos for this articleMexican-American War, also called Mexican War, Spanish Guerra de 1847 or Guerra de Estados Unidos a Mexico (“War of the United States Against Mexico”), 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.
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).
Fight for the title of War Wiz with this quiz on famous conflicts throughout history.
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.”
Learn about Abraham Lincoln and the Spot Resolutions
Learn more about Abraham Lincoln's Spot Resolutions during the Mexican-American War.
Encyclopædia Britannica, Inc.See all videos for this articleCongress 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.
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.
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.