He studied at Rutgers College in the U.S., took his military training in Germany, and became vice-minister of the Army, in the Meiji era, in 1892. In 1900, he also became governor-general of Formosa (i.e., Taiwan), simultaneously holding the Army Ministry for the fourth Hirabumi Ito government. Later, he was home minister in the first Katsura cabinet.
As the Russo-Japanese War approached, he took what was perceived as a lower-status post as vice chief of staff, and then served as chief of staff to Field Marshal Iwao Oyama, in which post he was considered the key planner. He advocated the surprise attack to pin the Russian fleet in Port Arthur as the armies invaded elsewhere, but also had the rare wisdom not to overextend his country, and recommended consolidating and suing for peace after major gains. 
- Robert Greene (2007), The 33 Strategies of War, Penguin, ISBN 9780143112785, p. 293
- "GEN. KODAMA DEAD.; He Was Field Marshal Oyama's Chief of Staff In the Russian War.", New York Times, 23 July 1906