Specializing as an intelligence officer, he was assigned to the Pacific Fleet, stationed on Guam and deployed to five different aircraft carriers with a photographic reconnaissance squadron in 1959–1961.
"[1] Hagan began his academic career as a part-time Instructor, San Bernardino Valley College on the Spring of 1965.
In 1969, he was appointed assistant professor in the History Department at Kansas State University, where he remained until 1973.
Naval Academy, 1985; Council Member and Secretary, North American Society for Oceanic History, 1976–1979; Editorial Advisory Board, Military Affairs, 1979–1982, and on the Membership Committee, Society for Historians of American Foreign Relations, 1976–1982.
As an academic, he received the Navy Superior Civilian Service Award, the North American Society for Oceanic History's John Lyman Book Award for the Best Book on the History of the United States Navy in 1978 for In Peace and War.