The MexicanAmerican War, also known in the United States as the Mexican War and in Mexico as the Intervencin estadounidense en Mxico (U.S. intervention in Mexico), was an armed conflict between the United States and Mexico from 1846 to 1848. It followed the 1845 U.S. annexation of Texas, which Mexico considered Mexican territory because it did not recognize the Velasco treaty signed by Mexican General Antonio Lpez de Santa Anna when he was a prisoner of the Texian Army during the 1836 Texas Revolution. The Republic of Texas was de facto an independent country, but most of its citizens wished to be annexed by the United States. Domestic sectional politics in the U.S. were preventing annexation since Texas would have been a slave state, upsetting the balance of power between Northern free states and Southern slave states. In the 1844 United States presidential election, Democrat James K. Polk was elected on a platform of expanding U.S. territory in Oregon and Texas. Polk advocated expansion by either peaceful means or armed force, with the 1845 annexation of Texas furthering that goal by peaceful means. However, the boundary between Texas and Mexico was disputed, with the Republic of Texas and the U.S. asserting it to be the Rio Grande and Mexico claiming it to be the more-northern Nueces River. Both Mexico and the U.S. claimed the disputed area and sent troops. Polk sent U.S. Army troops to the area; he also sent a diplomatic mission to Mexico to try to negotiate the sale of territory. U.S. troops' presence was designed to lure Mexico into starting the conflict, putting the onus on Mexico and allowing Polk to argue to Congress that a declaration of war should be issued. Mexican forces attacked U.S. forces, and the United States Congress declared war.Beyond the disputed area of Texas, U.S. forces quickly occupied the regional capital of Santa Fe de Nuevo Mxico along the upper Rio Grande, which had trade relations with the U.S. via the Santa Fe Trail between Missouri and New Mexico. U.S. forces also moved against the province of Alta California and then moved south. The Pacific Squadron of the U.S. Navy blockaded the Pacific coast farther south in the lower Baja California Territory. The Mexican government refused to be pressured into signing a peace treaty at this point, making the U.S. invasion of the Mexican heartland under Major General Winfield Scott and its capture of the capital Mexico City a strategy to force peace negotiations. Although Mexico was defeated on the battlefield, politically its government's negotiating a treaty remained a fraught issue, with some factions refusing to consider any recognition of its loss of territory. Although Polk formally relieved his peace envoy, Nicholas Trist, of his post as negotiator, Trist ignored the order and successfully concluded the 1848 Treaty of Guadalupe Hidalgo. It ended the war, and Mexico recognized the Mexican Cession, areas not part of disputed Texas but conquered by the U.S. Army. These were northern territories of Alta California and Santa Fe de Nuevo Mxico. The U.S. agreed to pay $15 million for the physical damage of the war and assumed $3.25 million of debt already owed by the Mexican government to U.S. citizens. Mexico acknowledged the independence of what became the State of Texas and accepted the Rio Grande as its northern border with the United States.

The victory and territorial expansion Polk envisioned inspired patriotism among some sections of the United States, but the war and treaty drew fierce criticism for the casualties, monetary cost, and heavy-handedness, particularly early on. The question of how to treat the new acquisitions also intensified the debate over slavery in the United States. Although the Wilmot Proviso that explicitly forbade the extension of slavery into conquered Mexican territory was not adopted by Congress, debates about it heightened sectional tensions. Some scholars see the MexicanAmerican War as leading to the American Civil War, with many officers trained at West Point, who saw action in Mexico, playing prominent leadership roles on each side during the conflict.

In Mexico, the war worsened domestic political turmoil. Since the war was fought on home ground, Mexico suffered a large loss of life of both its soldiers and its civilian population. The nation's financial foundations were undermined, territory was lost, and national prestige left it in what a group of Mexican writers including Ramn Alcaraz and Jos Mara del Castillo Velasco called a "state of degradation and ruin... This group did not acknowledge Mexicos refusal to admit the independence of Texas as a cause of the war, instead proclaiming [As for] the true origin of the war, it is sufficient to say that the insatiable ambition of the United States, favored by our weakness, caused it."

James Knox Polk (November 2, 1795 – June 15, 1849) was the 11th president of the United States, serving from 1845 to 1849. He previously was the 13th Speaker of the House of Representatives (1835–1839) and ninth governor of Tennessee (1839–1841). A protégé of Andrew Jackson, he was a member of the Democratic Party and an advocate of Jacksonian democracy. Polk is chiefly known for extending the territory of the United States through the Mexican–American War; during his presidency, the United States expanded significantly with the annexation of the Republic of Texas, the Oregon Territory, and the Mexican Cession following American victory in the Mexican–American War.

After building a successful law practice in Tennessee, Polk was elected to its state legislature in 1823 and then to the United States House of Representatives in 1825, becoming a strong supporter of Andrew Jackson. After serving as chairman of the Ways and Means Committee, he became Speaker of the House in 1835, the only person to have served both as Speaker and U.S. president. Polk left Congress to run for governor of Tennessee, winning in 1839 but losing in 1841 and 1843. He was a dark horse candidate in the 1844 presidential election as the Democratic Party nominee; he entered his party's convention as a potential nominee for vice president but emerged as a compromise to head the ticket when no presidential candidate could secure the necessary two-thirds majority. In the general election, Polk defeated Henry Clay of the rival Whig Party.

Historians have praised Polk for meeting every major domestic and foreign policy goal he had set during his single term. After a negotiation fraught with the risk of war, he reached a settlement with Great Britain over the disputed Oregon Country, the territory, for the most part, being divided along the 49th parallel. He provoked a war with Mexico in an attempt to expand the United States and succeeded in doing so, as it resulted in Mexico's cession of nearly all the American Southwest. He secured a substantial reduction of tariff rates with the Walker tariff of 1846. The same year, he achieved his other major goal, re-establishment of the Independent Treasury system. True to his campaign pledge to serve only one term, Polk left office in 1849 and returned to Tennessee, where he died three months after leaving the White House.

Though relatively obscure today, scholars have ranked Polk favorably for his ability to promote and achieve the major items on his presidential agenda, despite limiting himself to a single term. He has also been criticized for leading the country into an aggressive war against Mexico and thus exacerbating divides between free and slave states. A property owner who used slave labor for most of his adult life, he kept a plantation in Mississippi and increased his slave ownership while president. The legacy of Polk's presidency of territorial expansion – with the United States reaching the Pacific coast and, roughly, its present contiguous borders – was making the United States a nation poised to become a world power, but with sectional divisions gravely exacerbated, setting the stage for the Civil War.