In Mexico, Thomas led a gun crew with distinction at the battles of Fort Brown, Resaca de la Palma, Monterrey, and Buena Vista, receiving three brevet promotions. At Buena Vista, Gen. Zachary Taylor reported that "the services of the light artillery, always conspicuous, were more than unusually distinguished" during the battle. Brig. Gen. John E. Wool wrote about Thomas and another officer that "without our artillery we would not have maintained our position a single hour." Thomas's battery commander wrote that Thomas's "coolness and firmness contributed not a little to the success of the day. Lieutenant Thomas more than sustained the reputation he has long enjoyed in his regiment as an accurate and scientific artillerist." During the war, Thomas served closely with an artillery officer who would be a principal antagonist in the Civil War—Captain Braxton Bragg.