The Mexican–American War, also known in the United States as the Mexican War was an armed conflict between the United States and Mexico from 1846 to 1848. It followed the 1845 American annexation of Texas, which Mexico considered its territory. Mexico refused to recognize the Velasco treaty, signed by President Antonio López de Santa Anna when he was captured by the Texan Army during the 1836 Texas Revolution. The Republic of Texas was de facto an independent country, but most of its Anglo-American citizens wanted to be annexed by the United States.

Sectional politics over slavery in the United States were preventing annexation because Texas would have been admitted as 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 to Oregon, California (also a Mexican territory) and Texas; by any means, with the 1845 annexation of Texas furthering that goal. But 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. Polk sent a diplomatic mission to Mexico in an attempt to buy the disputed territory, together with California and everything in between for US $25 million; an offer the Mexican government refused. The U.S. sent troops to the disputed Rio Grande, ignoring Mexican demands to withdraw. Mexico subsequently attacked a group of 80 soldiers on April 25, 1846, a move which Polk used to convince the Congress of the United States to declare war.

The Siege of Fort Texas began on May 3, 1846. Mexican artillery at Matamoros opened fire on Fort Texas, which replied with its own guns. The bombardment continued for 160 hours and expanded as Mexican forces gradually surrounded the fort. Thirteen U.S. soldiers were injured during the bombardment, and two were killed. Among the dead was Jacob Brown, after whom the fort was later named.

On May 8, 1846, Zachary Taylor and 2,400 troops arrived to relieve the fort.[73] However, General Arista rushed north with a force of 3,400 and intercepted him about 5 miles north of the Rio Grande River, near modern-day Brownsville, Texas. The U.S. Army employed "flying artillery", their term for horse artillery, a mobile light artillery mounted on horse carriages with the entire crew riding horses into battle. The fast-firing artillery and highly mobile fire support had a devastating effect on the Mexican army. In contrast to the "flying artillery" of the Americans, the Mexican cannons at the Battle of Palo Alto had lower-quality gunpowder that fired at velocities slow enough to make it possible for American soldiers to dodge artillery rounds. The Mexicans replied with cavalry skirmishes and their own artillery. The U.S. flying artillery somewhat demoralized the Mexican side, and seeking terrain more to their advantage, the Mexicans retreated to the far side of a dry riverbed (resaca) during the night and prepared for the next battle. It provided a natural fortification, but during the retreat, Mexican troops were scattered, making communication difficult.

During the Battle of Resaca de la Palma on May 9, 1846, the two sides engaged in fierce hand-to-hand combat. The U.S. Cavalry managed to capture the Mexican artillery, causing the Mexican side to retreat—a retreat that turned into a rout. Fighting on unfamiliar terrain, his troops fleeing in retreat, Arista found it impossible to rally his forces. Mexican casualties were significant, and the Mexicans were forced to abandon their artillery and baggage. Fort Brown inflicted additional casualties as the withdrawing troops passed by the fort, and additional Mexican soldiers drowned trying to swim across the Rio Grande. Taylor crossed the Rio Grande and began his series of battles in Mexican territory.

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. U.S. forces also moved against the province of Alta California and then turned south. The Pacific Squadron of the U.S. Navy blockaded the Pacific coast in the lower Baja California Territory. The U.S. Army, under Major General Winfield Scott, invaded the Mexican heartland and captured the capital, Mexico City, in September 1847.

Although Mexico was defeated on the battlefield, negotiating peace was a politically fraught issue. Some Mexican factions refused 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 cession of present-day California, Nevada and Utah as well as parts of present-day Arizona, Colorado, New Mexico and Wyoming. 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 relinquished its claims on 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. The question of how to treat the new acquisitions 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. Many officers who had trained at West Point gained experience in the war in Mexico and later played prominent leadership roles during the Civil War.

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 soldiers and civilian population. The nation's financial foundations were undermined, and more than half of its territory was lost. Mexico felt a loss of national prestige, leaving 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."