While the immediate cause of the war was the U.S. annexation of Texas (Dec., 1845), other factors had disturbed peaceful relations between the two republics. In the United States there was agitation for the settlement of long-standing claims arising from injuries and property losses sustained by U.S. citizens in the various Mexican revolutions.
Another major factor was the American ambition, publicly stated by President Polk, of acquiring California, upon which it was believed France and Great Britain were casting covetous eyes. Despite the rupture of diplomatic relations between Mexico and the United States that followed congressional consent to the admission of Texas into the Union, President Polk sent John Slidell to Mexico to negotiate a settlement. Slidell was authorized to purchase California and New Mexico, part of which was claimed by Texas, and to offer the U.S. government's assumption of liability for the claims of U.S. citizens in return for boundary adjustments.
When Mexico declined to negotiate, the United States prepared to take by force what it could not achieve by diplomacy. The war was heartily supported by the outright imperialists and by those who wished slave-holding territory extended. The settlement of the Oregon boundary dispute (June, 1846), which took place shortly after the official outbreak of hostilities, seemed to indicate British acquiescence, for it granted the United States a free hand.
Early in May, 1845, American troops under Gen. Zachary Taylor had been stationed at the Sabine River preliminary to an advance to the Rio Grande, the southern boundary claimed by Texas. They advanced to Corpus Christi in July. In Mar., 1846, after the failure of Slidell's mission, Taylor occupied Point Isabel, a town at the mouth of the Rio Grande. To the Mexicans, who claimed the Nueces River as the boundary, this was an act of aggression, and after some negotiations Gen. Mariano Arista ordered his troops to cross the Rio Grande. On Apr. 25 a clash between the two armies occurred, and Taylor reported to Washington that hostilities had begun.
On May 3 the guns of Matamoros began to shell Fort Brown (then Fort Taylor), an advanced American position near the present Brownsville, Tex. President Polk called these Mexican actions an invasion of American soil, and on May 13, 1846, the United States declared war. Meanwhile, Taylor had defeated the Mexicans at Palo Alto (May 8) and Resaca de la Palma (May 9). The Mexicans retreated across the Rio Grande. Taylor followed them and on May 18 took Matamoros. After a delay he then advanced on Monterrey, which he occupied after a five-day battle (Sept. 20-24, 1846).
In June, 1846, Gen. Stephen W. Kearny left Fort Leavenworth for New Mexico with some 1,600 men, including a force of Missouri volunteers under Alexander Doniphan. Santa Fe was taken (August), a provisional government was set up, and Doniphan was placed in command of the area. Kearny pushed on to California to find that this province, through the agency of Commodore John D. Sloat (later relieved by Robert F. Stockton) and John C. Frémont, was already under American rule. After reinforcements reached Santa Fe, Doniphan invaded (Dec., 1846) N Mexico, taking El Paso and Chihuahua before he joined forces with Gen. John E. Wool (who had advanced southwest from San Antonio) and with Taylor at Saltillo.
Gen. Antonio López de Santa Anna, who had been in exile in Cuba and had been allowed passage through the U.S. blockade at Veracruz, had now assumed the presidency of Mexico; he gathered a large force to stop Taylor's advance. Taylor, whose army had been greatly reduced in size, was in an extremely vulnerable position when hit by Santa Anna in the battle of Buena Vista (Feb., 1847). The fighting was hard and appeared indecisive for a time, but in the end the Mexicans withdrew in confusion.
The final campaign of the war began with the landing of U.S. forces under Gen. Winfield Scott at Veracruz in Mar., 1847. Scott was supported by a naval task force under David Conner (who was relieved by Matthew C. Perry); they landed some 12,000 men and after a three-day bombardment took the city. Scott then began his drive on Mexico City. In April, Santa Anna was defeated at the mountain stronghold of Cerro Gordo. After hard fighting Mexican forces were also routed at Contreras and Churubusco (August).
On Aug. 24 the Mexicans accepted an armistice, but after two weeks of futile peace negotiations, fighting was resumed. The Mexican capital was heavily defended by garrisons at Casa Mata and Molino del Rey and by the great fortress of Chapultepec. William J. Worth carried Casa Mata and Molino del Rey, and the supposedly impregnable Chapultepec was stormed in a savage American assault led by Gen. John A. Quitman. On Sept. 14, 1847, American troops entered Mexico City, where they remained until peace was restored.
The United States had won an easy victory, partly because Mexico, torn by civil strife, could not present a united front to face the invader. The Mexican presidency had changed hands a number of times during the war, and some Mexican states had refused to cooperate with the central government. Peace negotiations were conducted on behalf of the United States by Nicholas P. Trist, a secret envoy, whose relations with General Scott were at first strained. Although recalled by President Polk, Trist decided to ignore the order and continue his negotiations, which resulted in the Treaty of Guadalupe Hidalgo (Feb. 2, 1848). By the terms of the treaty, Mexico ceded to the United States two fifths of its territory and received an indemnity of $15 million and the assumption of American claims against Mexico by the U.S. government. The boundary between the two countries, as outlined, was to follow the Rio Grande from its mouth to the New Mexico line, then run west to the Gila River, follow the Gila to the Colorado River and then follow the boundary between Upper California and Lower California to the Pacific.
See G. L. Rives, The United States and Mexico, 1821-1848 (1913, repr. 1969); J. H. Smith, The War with Mexico (1919, repr. 1963); B. De Voto, The Year of Decision (1943, repr. 1961); A. H. Bill, Rehearsal for Conflict (1947, repr. 1969); R. S. Henry, The Story of the Mexican War (1950, repr. 1961); O. A. Singletary, The Mexican War (1960); R. E. Ruiz, The Mexican War: Was It Manifest Destiny? (1963); D. M. Fletcher, The Diplomacy of Annexation (1973); K. J. Bauer, The Mexican War (1974); J. H. Schroeder, Mr. Polk's War (1974), G. N. Brack, Mexico Views Manifest Destiny (1975), and J. M. McPherson, Battle Cry of Freedom (1988).