A conflict that occurred as the result of Mexican resentment over the US annexation of Texas and a border dispute, the Mexican-American War represents the only major military dispute between the two nations. The war was fought primarily in northeastern and central Mexico and resulted in a decisive American victory. As a result of the war, Mexico was forced to cede its northern and western provinces, which today comprise a significant portion of the western United States.
The causes of the Mexican-American War can be traced back to Texas winning its independence from Mexico in 1836. For the next nine years, many in Texas favored joining the United States, however Washington did not take action due to fears of increasing sectional conflict and angering Mexico. In 1845, following the election of the pro-annexation candidate, James K. Polk, Texas was admitted to the Union. Shortly thereafter, a dispute began with Mexico over the southern border of Texas. Both sides sent troops to the area, and on April 25, 1846, a US cavalry patrol, led by Captain Seth Thornton, was attacked by Mexican troops. Following the “Thornton Affair,” Polk asked Congress for a declaration of war, which was issued on May 13.
On May 8, 1846, Brig. Gen. Zachary Taylor was moving to relieve Fort Texas, when he was intercepted at Palo Alto by Mexican troops under Gen. Mariano Arista. In the battle that ensued Taylor defeated Arista. The battle continued the next day at Resaca de la Palma, with Taylor's men driving the Mexicans back across the Rio Grande. Reinforced, Taylor advanced into Mexico and, following heavy fighting, captured Monterrey. When the battle ended, Taylor offered the Mexicans a two month truce in exchange for the city. This move angered Polk who began to strip Taylor’s army of men for use in invading central Mexico. Taylor's campaign ended in February 1847, when his 4,500 men won a stunning victory over 15,000 Mexicans at the Battle of Buena Vista.
In mid-1846, General Stephen Kearny was dispatched west with 1,700 men to capture Santa Fe and California. Meanwhile, US naval forces, commanded by Commodore Robert Stockton, descended on the coast of California. With the aid of American settlers, they swiftly captured the towns along the coast. In late 1846, they aided Kearny’s exhausted troops as they emerged from the desert and together forced the final surrender of Mexican forces in California.
On March 9, 1847, General Winfield Scott landed 10,000 men outside of Veracruz. After a brief siege, he captured the city on March 29. Moving inland, his forces defeated a larger Mexican army at Cerro Gordo. As Scott’s army neared Mexico City, they fought successful engagements at Contreras, Churubusco, and Molino del Rey. On September 13, 1847, Scott launched an attack on Mexico City itself, assaulting Chapultepec Castle and capturing the gates of the city. Following the occupation of Mexico City, the fighting effectively ended.
The war ended on February 2, 1848, with the signing of the Treaty of Guadalupe Hidalgo. This treaty ceded to the United States the land that now comprises the states of California, Utah, and Nevada, as well as parts of Arizona, New Mexico, Wyoming, and Colorado. Mexico also renounced all rights to Texas. During the war 1,773 Americans were killed in action and 4,152 were wounded. Mexican casualty reports are incomplete, but it estimated that approximately 25,000 were killed or wounded between 1846-1848.