Mexican forces opened artillery fire on Fort Texas (near modern Brownsville), initiating open hostilities along the disputed Nueces–Rio Grande borderlands and helping precipitate wider war.
The Battle of Palo Alto, the first major engagement of the conflict, was fought on contested ground in South Texas; U.S. forces under Zachary Taylor repelled Mexican troops under Mariano Arista.
U.S. forces defeated Mexican troops at Resaca de la Palma, helping secure the lower Rio Grande Valley for the U.S. Army early in the war.
After reporting clashes along the Rio Grande, the United States formally declared war on Mexico, transforming border fighting into a national war effort.
U.S. troops captured Monterrey after intense urban fighting; the outcome established U.S. momentum in northern Mexico and led to a negotiated evacuation and armistice.
The Capitulation (Treaty) of Cahuenga established a ceasefire between U.S. forces and Californio forces, effectively ending major combat in Alta California during the war.
At Buena Vista (La Angostura), Santa Anna’s larger army fought Zachary Taylor’s forces; the battle became one of the war’s best-known northern campaigns and checked Mexico’s offensive in that theater.
U.S. forces landed and began the Siege of Veracruz, a major amphibious operation that opened Winfield Scott’s campaign toward Mexico City.
Following the assault on Chapultepec, U.S. forces entered and occupied Mexico City, creating decisive pressure for a peace settlement.
U.S. and Mexican plenipotentiaries signed the Treaty of Guadalupe Hidalgo, ending the war and establishing a new boundary while transferring a vast northern Mexican cession to the United States in exchange for compensation and assumption of certain claims.
The U.S. Senate advised and consented to ratification with amendments, a key domestic step required before the treaty could take effect between the two states.
The United States and Mexico exchanged ratifications at Querétaro, completing the bilateral ratification process and clearing the way for formal proclamation and implementation.
The treaty was formally proclaimed, marking its entry into force and accelerating the political, legal, and administrative transitions in the newly ceded territories.
Congress enacted a package of measures addressing the status and governance of territories acquired after 1848, including admitting California and organizing Utah and New Mexico territories—an attempt to manage the sectional crisis intensified by the Mexican Cession.
Mexico and the United States signed an agreement for the Gadsden Purchase, a later boundary-and-territory adjustment that reshaped the southern border of the continental United States after the 1848 settlement.
After revisions (reducing the territory and price), the U.S. Senate ratified the new treaty text, enabling final completion of the purchase in 1854.
Mexican–American War and the Treaty of Guadalupe Hidalgo (1846–1850)
