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. When General Taylor counterattacked, the 2nd Dragoons forced the enemy to turn their flank during the Battle of Palo Alto. The next day, during the Battle of Resaca de la Palma on 9 May 1846, Companies D and E under Captain Charles A. May were ordered to eliminate a battery of Mexican guns. Prior to the charge, May issued a simple order; "Remember your Regiment and follow your officers." This became the 2nd Dragoon Regiment's motto. The attack destroyed the enemy battery and captured a Mexican general. On 29 June 1846, COL David Twiggs was given command of the regiment from COL William S. Harney, and he was lauded for his bravery at the Battle of Monterrey. COL Twiggs commanded the 2nd Dragoons for the rest of the war, and by the end, the regiment was one of two regiments in the Army that had elements participate in every major battle. Heroism was not limited to the officers of the 2nd Dragoons; in November 1847, SGT Jack Miller's small patrol of 20 Dragoons was ambushed by near Monclova by 100 Mexicans. Reaching for their carbines, SGT Miller urged them to charge with only their sabers. In the ensuing battle, 6 Mexicans were killed, 13 were wounded, and 70 were captured at the cost of 1 wounded Dragoon and 3 wounded horses. Frontier duty After the Mexican–American War, the 2nd Dragoons headed west to protect the settlers on the new frontier that had just been gained by the United States in the Treaty of Guadalupe Hidalgo. In June 1849, F Company, under MAJ Ripley Arnold, established Fort Worth along the Trinity River. These years were spent patrolling the frontier in order to protect American settlers heading west from hostile Indians. In 1854, the Companies E and K of the regiment defeated a sizable Sioux force in the Battle of Ash Hollow in Nebraska, forcing the Sioux to sign a peace treaty. In late 1857, in response to growing hostilities between federal authorities and Mormon settlers in Utah, a battalion of the 2nd Dragoons was sent to quell any Mormon resistance to federal power. These Dragoons, under LTC Philip St. George Cooke,