メキシコ共和国
In October 1842, A, D, E, F, and G Companies moved to Fort Jessup, Louisiana and Fort Towson. The remainder of the regiment stayed in Florida to patrol for hostile bands of Seminoles. Fort Jessup became the regimental headquarters, and was the 2nd Dragoons' home for four years. When hostilities with the Centralist Republic of Mexico began to boil over in 1845, General Zachary Taylor assembled his "Army of Observation" at Fort Jessup, and the 2nd Dragoons marched overland to occupy Corpus Christi, Texas. They soon established Fort Texas, near modern-day Brownsville, Texas. The regiment conducted aggressive patrolling along the Rio Grande, and on 25 April 1846, they received word that Mexican troops were crossing the river. Two companies of the 2nd Dragoons were ambushed by 500–1,600 Mexican troops (accounts vary), and all were either killed or captured. This battle, known as the Thornton Affair, gave US President Polk the "casus belli" he needed to invade Mexico.