The Mexican-American War (1846-1848) was an armed conflict between the United States and Mexico, triggered by disputed territorial claims following the U.S. annexation of Texas in 1845. The central dispute concerned whether the southern boundary of Texas lay at the Nueces River, as Mexico maintained, or the Rio Grande, as the United States claimed. The war ended with the Treaty of Guadalupe Hidalgo (2 February 1848), by which Mexico ceded approximately 525,000 square miles of territory - including present-day California, Nevada, Utah, most of Arizona and New Mexico, and parts of Colorado and Wyoming - in exchange for $15 million and U.S. assumption of claims against Mexico by American citizens.

Tensions between the United States and Mexico had been building for years before open hostilities. The Republic of Texas had declared independence from Mexico in 1836 after the Battle of San Jacinto, though Mexico never formally recognized Texan sovereignty. U.S. annexation of Texas in December 1845 was treated by Mexico as an act of aggression. President James K. Polk, an advocate of westward expansion, sent General Zachary Taylor to the disputed territory between the Nueces and the Rio Grande. After Mexican forces crossed the Rio Grande and engaged U.S. troops in April 1846, Polk asked Congress for a declaration of war, which was passed on 13 May 1846. Mexico declared war on the same day.

The broader ideological context included the concept of Manifest Destiny - the widely held belief that U.S. expansion to the Pacific was inevitable and divinely sanctioned. Critics at the time and since have characterized the war as a pretext for territorial acquisition. For a fuller treatment of causes and events, see Mexican-American War - History.

The United States prosecuted the war on multiple fronts. Taylor's forces won engagements at Palo Alto and Resaca de la Palma before capturing Monterrey. General Winfield Scott led an amphibious landing at Veracruz and a subsequent march to Mexico City, which fell in September 1847. Concurrently, U.S. forces under Stephen W. Kearny and others secured New Mexico and California. The Mexican military, though fighting on home territory, was hampered by internal political instability - Antonio Lopez de Santa Anna returned from exile during the war and assumed command of Mexican forces.

The treaty ending the war was negotiated by Nicholas Trist, a State Department official whom Polk had recalled but who completed negotiations on his own authority. Beyond the territorial cession and monetary payment, the treaty guaranteed property rights and citizenship options to Mexicans living in the ceded territories. Subsequent U.S. enforcement of those guarantees has been a source of ongoing historical and legal dispute. The Gadsden Purchase (1853) later added a further strip of territory along the present-day Arizona-New Mexico border.

The war was not without significant domestic opposition in the United States. Representative Abraham Lincoln challenged Polk's justification with his “Spot Resolutions,” demanding to know the exact spot where American blood had allegedly been shed on American soil. Henry David Thoreau's refusal to pay taxes in protest contributed to his essay Resistance to Civil Government. The war deepened sectional conflict over slavery, as any new territory raised immediate questions about whether it would enter the union as free or slave. The Wilmot Proviso, which would have banned slavery in any territory acquired from Mexico, passed the House but failed in the Senate and intensified those tensions.

Historians and commentators assess the war through several distinct lenses. Some view it as straightforward territorial aggrandizement by a stronger power against a weaker neighbor, with the border dispute serving as a manufactured casus belli. Others emphasize the role of Manifest Destiny ideology and its relationship to American slavery as an institution seeking room to expand. Still others focus on the war's constitutional and political implications, particularly the shift in war-making authority and the downstream effects on the sectional crisis that produced the Civil War. Mexican historiography has traditionally treated the war as an unjust invasion; American historiography has varied considerably by era and political context.

The war also raises questions about the treatment of Mexican citizens who became American nationals under the treaty, whose land grants were frequently litigated away in subsequent decades - a grievance that informs ongoing debates about U.S.-Mexico relations and the political identity of Mexican Americans.

There is broad historical consensus on the basic sequence of events, the terms of the treaty, and the territorial outcome. The moral and political characterization of the war - whether it was justified, whether the border dispute was manufactured, and what obligations the treaty imposed on the United States - remains contested among historians and commentators. See Mexican-American War - Historical Consensus.

• Justified Expansion - The war secured legitimate territorial claims and fulfilled the natural westward development of the United States.
• Imperial Aggression - The war was an unprovoked land grab prosecuted under a fabricated pretext, in violation of Mexican sovereignty.
• War for Slavery's Extension - The war's primary driver was the political and economic interest of slaveholding states seeking new territory.
• Manifest Destiny Critique - The war illustrates how ideological frameworks naturalize conquest and obscure its moral costs.

• Mexican-American War - History
• Mexican-American War - Debate
• Mexican-American War - Historical Consensus
• Manifest Destiny
• Texas Annexation
• Treaty of Guadalupe Hidalgo
• Wilmot Proviso
• Gadsden Purchase
• American Civil War
• Slavery in the United States

1. Merk, Frederick. Manifest Destiny and Mission in American History. Harvard University Press, 1963.
2. Howe, Daniel Walker. What Hath God Wrought: The Transformation of America, 1815-1848. Oxford University Press, 2007.
3. Henderson, Timothy J. A Glorious Defeat: Mexico and Its War with the United States. Hill and Wang, 2007.
4. Bauer, K. Jack. The Mexican War, 1846-1848. Macmillan, 1974.
5. Eisenhower, John S.D. So Far from God: The U.S. War with Mexico, 1846-1848. Random House, 1989.
6. Treaty of Guadalupe Hidalgo, 2 February 1848. 9 Stat. 922.
7. Thoreau, Henry David. “Resistance to Civil Government.” Aesthetic Papers, 1849.