Mexico obtained independence from Spain and the Spanish Empire with the Treaty of Córdoba in 1821. It briefly experimented with monarchy, but became a republic in 1824. This government was characterized by instability, leaving it ill-prepared for international conflict when war broke out only two decades later, in 1846.
In the decades preceding the war, Native American raids in Mexico’s sparsely settled north prompted the Mexican government to sponsor migration from the United States to the Mexican province of Texas to create a buffer. However, the newly named “Texans” revolted against the Mexican government of President/dictator Antonio López de Santa Anna, who had usurped the Mexican Constitution of 1824, in the subsequent 1836 Texas Revolution, creating a republic not recognized by Mexico, which still claimed it as part of its national territory.
In 1845, the Texan Republic agreed to an offer of annexation by the U.S. Congress and became the 28th state in the Union on December 29 that year.
Mexico’s military and diplomatic capabilities declined after it attained independence from Spain in 1821 and left the northern one-half of the country vulnerable to the Comanche, Apache, and Navajo native Americans. The Comanche, in particular, took advantage of the Mexican state to undertake large-scale raids hundreds of miles into the country to acquire livestock for their own use and to supply an expanding market in Texas and the U.S.
The northern area of Mexico was sparsely settled and not well controlled politically by the government based in Mexico City. After independence, Mexico contended with internal struggles that sometimes verged on civil war and the northern frontier was not a high priority. In northern Mexico, the end of Spanish rule was marked by the end of financing for presidios and for gifts to Native Americans to maintain the peace. The Comanche and Apache were successful in raiding for livestock and looting much of northern Mexico outside the scattered cities. Northern Mexico was a violent and chaotic area due to the Indian raids. The raids after 1821 resulted in the death of thousands of Mexicans, halted most transportation and communications, and decimated the ranching industry that was a mainstay of the northern economy. As a result, the demoralized civilian population of northern Mexico put up little resistance to the invading U.S. army.
Distance and hostile activity from Native Americans also made communications and trade between the heartland of Mexico and provinces such as Alta California and New Mexico difficult. As a result, New Mexico was dependent on the overland Santa Fe Trail trade with the United States at the outbreak of the Mexican–American War.
The Mexican government’s policy of settlement of US citizens in its province of Tejas was aimed at expanding control into Comanche lands, the Comancheria. Instead of settlement occurring in the central and west of the province, people settled in East Texas, where there was rich farmland and which was contiguous to southern US slave states. As settlers poured in from the US, the Mexican government discouraged further settlement, with its 1829 abolition of slavery.
In 1836, Mexico was relatively united in refusing to recognize the independence of Texas. Mexico threatened war with the United States if it annexed the Republic of Texas. Meanwhile, U.S. President Polk’s assertion of Manifest Destiny was focusing United States interest on westward expansion beyond its existing national borders.
Mexican-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).
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.”
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 favored 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.
When war broke out, former Mexican president and general Antonio López de Santa Anna (the vanquisher of the Texan forces at the Alamo in 1836) contacted Polk. The U.S. president arranged for a ship to take Santa Anna from his exile in Cuba to Mexico for the purpose of working for peace. Instead of acting for peace, however, on his return, Santa Anna took charge of the Mexican forces.
Following its original plan for the war, the United States sent its army from the Rio Grande, under Taylor, to invade the heart of Mexico while a second force, under Col. Stephen Kearny, was to occupy New Mexico and California. Kearny’s campaign into New Mexico and California encountered little resistance, and the residents of both provinces appeared to accept U.S. occupation with a minimum of resentment. Meanwhile, Taylor’s army fought several battles south of the Rio Grande, captured the important city of Monterrey, and defeated a major Mexican force at the Battle of Buena Vista in February 1847. But Taylor showed no enthusiasm for a major invasion of Mexico, and on several occasions he failed to pursue the Mexicans vigorously after defeating them. In disgust, Polk revised his war strategy. He ordered Gen. Winfield Scott to take an army by sea to Veracruz, capture that key seaport, and march inland to Mexico City. Scott took Veracruz in March after a siege of three weeks and began the march to Mexico City. Despite some Mexican resistance, Scott’s campaign was marked by an unbroken series of victories, and he entered Mexico City on September 14, 1847. The fall of the Mexican capital ended the military phase of the conflict.
Ultimately, infection and disease took many more U.S. casualties than combat did. At least 10,000 troops died of illness, whereas some 1,500 were killed in action or died of battle wounds (estimates of the war’s casualties vary). Poor sanitation contributed to the spread of illness, with volunteers—who were less disciplined in their sanitary practices than regular troops were—dying in greater numbers than the regulars. Yellow fever was particularly virulent, but other diseases—such as measles, mumps, and smallpox—took their toll too, especially on troops from rural environments whose immunities were less developed than those of their urban compatriots.
Polk had assigned Nicholas Trist, chief clerk in the State Department, to accompany Scott’s forces and to negotiate a peace treaty. But after a long delay in the formation of a new Mexican government capable of negotiations, Polk grew impatient and recalled Trist. Trist, however, disobeyed his instructions and on February 2, 1848, signed the Treaty of Guadalupe Hidalgo. According to the treaty, which was subsequently ratified by both national congresses, Mexico ceded to the United States nearly all the territory now included in the 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.
Zachary Taylor emerged as a national hero and succeeded Polk as president in 1849. The war reopened the slavery-extension issue, which had been largely dormant since the Missouri Compromise. 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. The status of slavery in the newly acquired lands was eventually settled by the Compromise of 1850, but only after the nation had come perilously close to civil war. When the Civil War came in 1861, many of the most-noteworthy generals on both sides had profited from their battle experience in the Mexican-American War, including Confederate Generals Robert E. Lee, Thomas (“Stonewall”) Jackson, James Longstreet, George Pickett, Albert Sidney Johnston, Lewis Armistead, and P.G.T. Beauregard, as well as Union Generals Ulysses S. Grant (who later called the Mexican War “one of the most unjust ever waged by a stronger against a weaker nation”), George Gordon Meade, George H. Thomas, and Joseph Hooker.
In Mexico the war discredited the conservatives but left a stunned and despondent country. It also reinforced the worst stereotypes each country held about the other. Normalization of relations after the war proceeded slowly.