Smith served during the Mexican-American War, and was wounded in the Battle of Molino del Rey on September 8, 1847, one of the conflict's bloodiest engagements. After the war with Mexico ended he was promoted to first lieutenant on August 18, 1851. His final promotion in the U.S. Army came on June 4, 1858, to the rank of captain. He was in Europe on a leave of absence from the Army from 1859 to 1861.