2. Mexican-American War (1846)
On May 13, 1846, the U.S. Congress declared war on Mexico following disputes over Texas and the Rio Grande (history.com). The conflict, rooted in territorial expansion and the doctrine of Manifest Destiny, led to the acquisition of vast southwestern territories. The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, ended the war, with Mexico ceding over half its territory to the U.S.
