Dreben enlisted on June 27, 1899 in the 14th Infantry Regiment and was shipped to the Philippines (acquired by the U.S. as a result of its victory in the Spanish–American War) to help put down a native insurrection led by Emilio Aguinaldo. He quickly distinguished himself in battle. Later, he participated in the rescue of westerners besieged in Beijing during the Boxer Rebellion. Mustered out in 1902, he took a succession of unsatisfactory jobs, including an attempt to fight for the Japanese in the Russo-Japanese War, before reenlisting in 1904. This time, he was stationed at Fort Bliss. It was here that he was trained how to use a machine gun, a skill for which he became well-known (and would need in later years). He made friends in nearby El Paso, Texas before his second army hitch ended in 1907.