The Mexican-American War

By the end of this lesson, students will be able to:

• Account for the causes of the Mexican-American War, including the annexation of Texas in 1845, the disputed boundary between the Nueces and the Rio Grande, and the expansionist agenda James K. Polk carried into the White House in 1845.
• Trace the military course of the war from the Thornton Affair on the disputed strip in April 1846 through the campaigns of Zachary Taylor in northern Mexico, Stephen Watts Kearny across New Mexico and into California, and Winfield Scott from Veracruz to Mexico City.
• Account for the territorial settlement reached in the Treaty of Guadalupe Hidalgo of 1848 and identify the present-day states carved from the Mexican Cession.
• Account for the Wilmot Proviso of 1846 as the moment the politics of slavery in the territories realigned along sectional rather than partisan lines.
• Account for the long-run consequences of the war, including the Gadsden Purchase of 1853, the California gold rush, and the deepening conflict over the future of slavery that the Mexican Cession produced.

The Mexican-American War of 1846 to 1848 was the largest territorial transfer in the continental history of the United States outside of the Louisiana Purchase, and the only foreign war in which the United States invaded a neighboring republic, occupied its capital, and dictated the terms of peace at the conqueror's table. Three threads converged on the war's outbreak. First, the annexation of Texas in 1845 brought into the federal Union a republic that Mexico had never recognized as independent and treated the annexation as an act of aggression. Second, the United States and Mexico disagreed sharply about where the new southern boundary lay: the United States claimed the Rio Grande, Mexico claimed the Nueces River roughly 150 miles to the north, and the disputed strip between the two rivers belonged in fact to neither government. Third, the new president James K. Polk had been elected in 1844 on the most aggressive expansionist platform any major American party had yet offered (KC-5.1.I.C). Polk wanted California and the Southwest, he was prepared to buy them or take them, and he ordered General Zachary Taylor into the disputed strip in early 1846 to force the issue.

The war began on April 25, 1846, when a Mexican cavalry unit attacked a small American patrol on the disputed northern bank of the Rio Grande. Polk had already drafted a war message arguing that the United States ought to fight Mexico over unpaid claims; the Thornton Affair allowed him to rewrite the message around the more politically useful claim that Mexico had "shed American blood upon the American soil." Congress declared war on May 13, 1846, by a vote of 174 to 14 in the House and 40 to 2 in the Senate. The war was politically popular in the South and West, where annexation and expansion had been Democratic articles of faith for a decade, and politically suspect among Northern Whigs, who saw the war as a war of conquest waged on behalf of a future slave power. A first-term Whig representative from Illinois, Abraham Lincoln, introduced the Spot Resolutions on December 22, 1847, asking Polk to identify the precise spot on which American blood had been shed and to prove that the spot was American soil. The resolutions never produced an answer, and the wartime opposition they sharpened cost Lincoln his House seat at the next election.

Three American campaigns produced the military victory. Zachary Taylor crossed the Rio Grande and defeated Mexican forces at Palo Alto on May 8, 1846, and at Resaca de la Palma the next day, then drove south to win a hard-fought victory at Buena Vista in February 1847 against a Mexican army roughly three times the size of his own. Stephen Watts Kearny led the Army of the West out of Fort Leavenworth in June 1846, occupied Santa Fe without resistance, and continued to California, where the Bear Flag Revolt of American settlers had already begun and where the United States Navy had seized Monterey and San Francisco. The decisive blow was Winfield Scott's amphibious landing at Veracruz in March 1847, the largest the United States military would mount until the Second World War, followed by a 250-mile march inland over Mexican fortifications at Cerro Gordo, Contreras, and Churubusco, the storming of Chapultepec Castle on September 13, 1847, and the occupation of Mexico City the following day. The Whig veterans who came home from these campaigns, Taylor and Scott above all, returned as national figures; Taylor would be elected president in 1848 on the strength of his battlefield record (KC-5.1.I.C).

The war's territorial consequences arrived with the Treaty of Guadalupe Hidalgo, signed in the Mexico City suburb of Villa de Guadalupe Hidalgo on February 2, 1848, and ratified by the Senate the following month. Mexico ceded roughly 525,000 square miles of land that today comprises California, Nevada, Utah, the bulk of Arizona and New Mexico, and parts of Colorado and Wyoming. Mexico also formally accepted the Rio Grande as the southern boundary of Texas. In exchange the United States paid Mexico fifteen million dollars and assumed roughly 3.25 million dollars of unpaid claims that American citizens held against the Mexican government. Articles VIII and IX of the treaty pledged the United States to protect the property of the Mexican nationals living in the ceded territory and to extend full citizenship to those who chose not to relocate south of the new border (KC-5.1.II.C). The promise was honored unevenly across the Southwest over the following decades; Mexican-American land grants confirmed by Spanish and Mexican law were stripped from their holders by Anglo courts and squatters in California and New Mexico, and the legal status of Mexican-American communities became one of the longest-running civil-rights questions in the federal system.

The political reckoning began before the treaty was signed. On August 8, 1846, four months into the war, Pennsylvania Democrat David Wilmot offered a one-sentence amendment to a routine wartime appropriations bill: that slavery would be banned from any territory the United States acquired from Mexico. The Wilmot Proviso passed the House twice on votes that broke cleanly along sectional rather than partisan lines and was blocked in the Senate, where slaveholding states held an even share of seats. The proviso never became law, but it accomplished something more lasting: it demonstrated that the question of slavery in the new territories now divided the country geographically and that no party loyalty would hold the issue together. Northern Democrats who had voted against earlier antislavery measures voted for the proviso; Southern Whigs who had opposed the war voted against the proviso. The political coalitions of the Second Party System were already beginning to crack along the fault line the Mexican Cession had opened (KC-5.1.II.C).

Two epilogues complete the lesson. In December 1853, Franklin Pierce's administration negotiated the Gadsden Purchase, which acquired roughly 30,000 additional square miles in present-day southern Arizona and New Mexico for ten million dollars. The strip mattered because it offered the most level southern route for a transcontinental railroad and because Southern Democrats wanted to ensure that any Pacific railroad ran through slave territory. The purchase set the southern boundary of the continental United States in its present form. The second epilogue is the gold rush of 1849. James Marshall's discovery of gold at Sutter's Mill on the American River in January 1848, only nine days before the Treaty of Guadalupe Hidalgo was signed, set off the largest single-year migration in nineteenth-century American history. By the end of 1849 California's non-Indigenous population had quadrupled, and the territory's application to enter the Union as a free state in 1850 forced Congress to confront the question the Wilmot Proviso had posed four years earlier. The chain of events that runs from Polk's war message through Wilmot, Compromise, Kansas-Nebraska, and Dred Scott to the secession crisis of 1860 begins with the territorial settlement reached at Guadalupe Hidalgo.

A causal lesson works best when it samples sources that mark the war's opening, the debate over its purpose, the treaty that ended it, and the political consequence that opened the next decade of crisis.

1. (Causation) Account for the relative weight of the three causes that produced the Mexican-American War: the annexation of Texas in 1845, the boundary dispute between the Nueces and the Rio Grande, and Polk's expansionist agenda. Which of the three is the most proximate cause, and which is the most fundamental cause, and why?
2. (Sourcing) Compare Polk's war message of May 11, 1846, with Lincoln's Spot Resolutions of December 22, 1847. How does each text construct the factual record on which it rests its argument? Which is the more persuasive document on its own internal evidence, and why?
3. (Causation) The Wilmot Proviso never became law. Account for why historians nevertheless treat it as one of the most consequential pieces of failed legislation in the Period 5 sequence. What did its passage in the House and its failure in the Senate reveal about the political coalitions of 1846?
4. (Comparison) Compare the Treaty of Guadalupe Hidalgo with the Louisiana Purchase of 1803 as territorial settlements. In what respects do the two acquisitions resemble each other, and in what respects do they diverge in scale, in legal complexity, and in political consequence?
5. (Causation) Account for the long-run effect of the Mexican Cession on the population of the modern Southwest. How did Articles VIII and IX of the Treaty of Guadalupe Hidalgo shape the legal and civil status of Mexican-American communities, and how did practice diverge from text in the decades after 1848?

Draft alignment — pending educator review. AP USH codes correspond to the official College Board Course and Exam Description (Effective Fall 2023, Version 1). Statements below are paraphrased in the CountryReports voice; refer to the College Board's published CED for verbatim wording.