In May 1846, Buckner resigned his teaching position to fight in the Mexican–American War, enlisting with the 6th U.S. Infantry Regiment. His early duties included recruiting soldiers and bringing them to the Texas border. In November 1846, he was ordered to join his company in the field; he met them en route between Monclova and Parras. The company joined John E. Wool at Saltillo. In January 1847, Buckner was ordered to Vera Cruz with William J. Worth's division. While Maj. Gen. Winfield Scott besieged Vera Cruz, Buckner's unit engaged a few thousand Mexican cavalry at a nearby town called Amazoque.