The Mexican-American War (1846-1848) was a significant conflict driven by U.S. expansionist ambitions and Mexico’s political instability. The roots of the war trace back to the Texas Revolution of 1836, when Texas declared independence from Mexico. Mexico did not recognize Texas as independent and considered it still its territory. The U.S. was initially divided over annexing Texas due to concerns about disrupting the balance between free and slave states. However, in 1845, President James K. Polk, a proponent of "Manifest Destiny," pushed for the annexation of Texas and sought to acquire additional territories, including California and Oregon. Tensions over the boundary between Texas and Mexico intensified, with the U.S. claiming the Rio Grande and Mexico insisting on the Nueces River. After a failed attempt to purchase the disputed lands, Polk sent General Zachary Taylor’s troops to the area. The skirmish known as the Thornton Affair in April 1846, in which Mexican and U.S. forces clashed, was used by Polk to convince Congress to declare war.
Early engagements and major battles
In early 1846, President Polk sent General Zachary Taylor to the Rio Grande, defying Mexican demands to withdraw. Taylor established a fort on the disputed riverbank, later known as Fort Brown or Fort Texas. On April 25, Mexican cavalry attacked a U.S. patrol led by Captain Seth Thornton, resulting in the death of 11 American soldiers and the capture of 52. This incident, known as the Thornton Affair, significantly escalated tensions. On May 3, Mexican forces laid siege to Fort Texas, bombarding it for over 160 hours. U.S. casualties included Jacob Brown, after whom the fort was named. General Taylor, with 2,400 troops, relieved the fort on May 8 and engaged Mexican General Arista’s forces in the Battle of Palo Alto. The U.S. forces, utilizing superior artillery, forced a Mexican retreat. The following day, the Battle of Resaca de la Palma saw intense hand-to-hand combat, with U.S. cavalry capturing Mexican artillery and driving the Mexican forces into a rout. This defeat forced the Mexicans to abandon their equipment and suffer further losses during their retreat across the Rio Grande.
Expansion of the conflict
President Polk, viewing these incidents and Mexico’s refusal to negotiate as acts of war, requested a formal declaration from Congress, which was approved on May 13, 1846. Despite significant opposition from Whigs and abolitionists who feared the war was a pretext to expand slavery, many Americans supported the conflict, influenced by Manifest Destiny and concerns over British interests in the contested regions. U.S. military campaigns soon expanded across different areas. General Stephen W. Kearny advanced into New Mexico and California. In New Mexico, Kearny’s forces faced minimal resistance and established American control, though they contended with the Taos Revolt. In California, American settlers preemptively seized Mexican outposts in the Bear Flag Revolt. U.S. forces under Commodore John D. Sloat and later Commodore Robert F. Stockton secured California by early 1847, overcoming resistance, including the Siege of Los Angeles.
Santa Anna and the Pacific Coast
Mexican General Antonio López de Santa Anna, initially exiled, returned to Mexico promising to negotiate peace. Instead, he assumed command of the Mexican army and led it into battle. At the Battle of Buena Vista in February 1847, despite a large Mexican force, Santa Anna’s troops were repelled by U.S. forces, including the Mississippi Rifles. Santa Anna’s defeat at Buena Vista, coupled with internal political turmoil, weakened his position, though he assumed the Mexican presidency soon after. In the Pacific Coast campaign, U.S. naval forces captured La Paz and destroyed the Mexican fleet at Guaymas by October 1847. They cleared the Gulf of California of hostile vessels and captured Mazatlán by November 11, 1847. Mexican counter-campaigns, led by Manuel Pineda Muñoz, faced challenges due to U.S. naval artillery support. A significant victory came on March 31, 1848, with the defeat of Pineda’s forces at the Skirmish of Todos Santos, shortly after the Treaty of Guadalupe Hidalgo was signed.
Key campaigns and the fall of Mexico City
In northeastern Mexico, General Zachary Taylor faced challenges in capturing Monterrey, leading to an armistice that was later broken by Taylor to capture Saltillo. Taylor’s victory at Buena Vista bolstered his political career but led to criticism and the reassignment of troops. In northwestern Mexico, U.S. forces led by Alexander W. Doniphan faced minimal resistance, winning battles such as El Brazito and occupying Chihuahua City by March 1, 1847. Despite plans to advance to Mexico City, Doniphan was redirected to support Taylor. Southern Mexico saw U.S. naval blockades and control of the coast, particularly in the Gulf of Mexico. The U.S. Navy’s shallow-draft ships operated effectively, and despite some battles in Tabasco, U.S. forces refrained from intervening in the Caste War of Yucatán. General Winfield Scott led the campaign into Mexico’s heartland, beginning with an amphibious landing at Veracruz on March 9, 1847. After a fierce 12-day siege, Veracruz fell to U.S. forces. Scott’s army then advanced toward Mexico City, overcoming Mexican defenses at Cerro Gordo on April 18, 1847, and continuing to Puebla. After capturing Puebla, Scott’s forces faced guerrilla warfare but eventually succeeded in capturing Mexico City through battles such as Contreras and Churubusco. The fall of Mexico City on September 14, 1847, marked a significant victory, with General Scott receiving national acclaim. Santa Anna’s final campaign involved a failed siege of Puebla, which was lifted by U.S. forces on October 12, 1847. Following the capture of Mexico City, U.S. forces faced ongoing guerrilla resistance until the formal cessation of hostilities with the Treaty of Guadalupe Hidalgo in March 1848. Despite the treaty, some resistance persisted until the final U.S. evacuation in August.
Peace terms and aftermath
The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, officially ended the war. The treaty granted the U.S. control over Texas, established the Rio Grande as the border, and ceded territories including California, Nevada, Utah, and parts of several other states. In return, Mexico received $15 million, and the U.S. assumed $3.25 million in debts owed by Mexico to American citizens. The treaty was ratified by the U.S. Senate on March 10 and by Mexico on May 19. The Mexican Cession reduced Mexican territory by over 50%, leaving Mexico with just under 800,000 square miles. While the treaty promised U.S. citizenship, many Mexicans, especially non-white and indigenous peoples, faced discrimination and loss of rights. The war intensified debates over slavery and had a lasting impact on U.S. military leaders who later played significant roles in the Civil War.
Legacy
After the French departure from Mexico in 1867 and the reinstatement of a liberal republic, Mexico began addressing the war's aftermath. The story of the Niños Héroes, young cadets who died heroically at Chapultepec, became a central national narrative. Their sacrifice was formally recognized in 1881 with the construction of a cenotaph at Chapultepec, and the Monument to the Heroic Cadets was inaugurated in 1952. In 1981, the Museo Nacional de las Intervenciones was opened to detail past conflicts, including the 1846-1848 U.S. intervention. In the U.S., the war gradually faded from collective memory, but some commemorations persisted, including a monument in South Carolina and a painting in the Capitol. The Marine Corps Hymn references the war, and the Mexico City military cemetery holds the remains of U.S. soldiers. The Mormon Battalion's contributions were also recognized with monuments in Utah and Los Angeles.
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What event in 1836 contributed to the tensions leading up to the Mexican-American War?
