The son of a career army officer, Drum was attending Boston College when his father was killed during the Spanish–American War. Offered a direct commission in the United States Army, Drum was appointed a second lieutenant of Infantry. He served in the Philippines during the Philippine–American War, took part in the Battle of Bayan, and received the Silver Star for heroism. He continued to advance through positions of more rank and responsibility in the early 1900s, and took part in the Veracruz and Pancho Villa Expeditions.