War between the United States and Mexico stemming from Mexican anger at the United States' annexation of Texas in 1845 and from a dispute over whether Texas ended at the Nueces River (Mexican claim) or the Rio Grande (U.S. claim).

The war - in which U.S. forces were consistently victorious - resulted in the United States' acquisition of more than 500,000 square miles (1,300,000 square km) of Mexican territory extending westward from the Rio Grande to the Pacific Ocean.