Truman worked on the Manhattan Project under J. Robert Oppenheimer during World War II, working out of London while holding a commission with the United States Army Air Corps. His wife, whom he had married in 1942, was killed during a bombing raid in 1944, one year after accepting his commission.
Truman later worked with Edward Teller in the development of the hydrogen bomb, and by 1968 was part of Project Hercules, an effort to develop enhanced fusion technology. While working on the project, Truman served as a go-between for Soviet agents attempting to compromise the Hercules test, but then later tried to stop Dr. Diana Winters from carrying through with this foreign interference. (TOS - Assignment: Earth comic: "Brighter Than a Thousand Suns")
Edit