The Mexican–American War, also known in the United States as the Mexican War and in Mexico as the Intervención estadounidense en México (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 since the Mexican government did not recognize the Velasco treaty signed by Mexican General Antonio López 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 River 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 México 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 México. 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 Mexican–American 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 Ramón Alcaraz and José María del Castillo Velasco called a "state of degradation and ruin...” This group did not acknowledge Mexico’s 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."