Mexican cavalry attacked and overwhelmed a U.S. dragoon patrol led by Capt. Seth Thornton near the Rio Grande in disputed territory. U.S. President James K. Polk cited the clash as a key justification for asking Congress for a declaration of war.
U.S. forces under Gen. Zachary Taylor fought Mexican troops under Gen. Mariano Arista in the first major battle of the war. The engagement helped shape early momentum along the Rio Grande frontier.
Taylor’s army attacked Arista’s defensive position at Resaca de la Palma the day after Palo Alto, forcing Mexican forces to retreat south of the Rio Grande and consolidating U.S. control in the immediate border zone.
The United States formally declared war after Polk’s request to Congress, framing the conflict as a response to attacks on U.S. forces near the Rio Grande. This action marked the official start of the war between the two governments.
U.S. forces under Taylor fought Mexican defenders in and around Monterrey in intense urban combat. The battle ended with the city’s surrender and a negotiated armistice that became politically controversial in the United States.
Gen. Antonio López de Santa Anna’s larger army engaged Taylor’s smaller U.S. force near Buena Vista (La Angostura). The battle was tactically inconclusive but helped keep northern Mexico from being retaken by Mexican forces during this phase of the war.
U.S. forces under Gen. Winfield Scott landed near Veracruz and besieged the major Gulf port, establishing a crucial beachhead for the campaign toward Mexico City. The siege ended with Veracruz’s surrender later in March.
Scott’s army defeated Santa Anna’s forces at Cerro Gordo, outflanking strong Mexican positions and capturing prisoners and materiel. The victory facilitated the U.S. advance into central Mexico toward the capital.
U.S. forces defeated Mexican troops at Contreras (Padierna) southwest of Mexico City, gaining control of key approaches to the capital. The battle began the final phase of major operations around Mexico City.
A major engagement near Mexico City ended in U.S. victory and left Scott’s army positioned close to the capital. The battle is also noted for the involvement of the San Patricio Battalion fighting on the Mexican side.
U.S. troops attacked Mexican positions at Molino del Rey in a costly fight on the western approaches to Mexico City. The engagement was among the bloodiest of the campaign and preceded the assault on Chapultepec.
U.S. forces assaulted Chapultepec Castle, a strategic high point guarding access to Mexico City. The fall of Chapultepec helped open the way to the city’s capture immediately afterward.
Scott’s army entered Mexico City, effectively ending large-scale combat operations around the capital and accelerating negotiations to terminate the war. The occupation became the immediate backdrop for drafting and concluding a peace treaty.
U.S. and Mexican representatives signed the Treaty of Guadalupe Hidalgo, ending the war. The agreement set the border at the Rio Grande (and further west along the Gila River), confirmed major territorial cessions to the United States, and provided for U.S. payment to Mexico.
The U.S. Senate advised ratification of the treaty, but with amendments—most notably affecting provisions related to land grants. This step was required for the treaty to become binding under U.S. constitutional procedures.
The U.S. and Mexico exchanged ratifications of the Treaty of Guadalupe Hidalgo at Querétaro, completing the mutual approval process. The exchange marked the treaty’s entry into force in practice between the two states.
The treaty was formally proclaimed, publicly confirming its terms and the end of the war. Proclamation helped initiate implementation, including territorial transfer and boundary procedures.
Mexican–American War and the Treaty of Guadalupe Hidalgo (1846–1848)
