Mexican-American War, also known as the Mexican War, Spanish Guerra de 1847 or Guerra de Estados Unidos a Mexico (“America’s War against Mexico”), War between the United States and Mexico (April 1846-February 1848) Texas The state was assigned to Texas by the United States in 1845, and stemmed from a dispute over whether Texas was on the Newses River (claimed by Mexico) or the Rio Grande (claimed by the United States).
The war — which the U.S. military has consistently won — resulted in the United States gaining more than 500,000 square miles (1,300,000 square kilometers) of Mexican territory, which stretched westward from the Rio Rio Grande to the Pacific Ocean.
On May 9, 1846, Polk began preparing to send a war message to Congress to justify hostilities, citing Mexico’s refusal to pay U.S. claims and to negotiate with Slidell.
That night, he received word that Mexican troops had crossed the Rio Rio Grande on April 25 and attacked Taylor’s army, killing or wounding 16 people. In a quickly revised message of war, presented to Congress on May 11, Polk claimed that Mexico had “encroached upon our territory and shed American blood on American soil.”
PIC: Britannica, ThoughtCo, teaching American history, NDU Press-National Defense University