Upon graduation in 1942, Agawa was conscripted to serve in the Imperial Japanese Navy, where he worked as an intelligence officer breaking Chinese military codes until the end of the war.

August 6 as Agawa notes in a postscript, combines the stories of friends and acquaintances who experienced the bombing into the testimony of one family.

Occupation censorship at the time was strict, but the story passed because, the author later observed, "it made no reference to the problems of after-effect and continued no overt criticism of the U.S." Agawa came to popular and critical attention with his Citadel in Spring (春の城, 1952), which was awarded the Yomiuri Prize.

Ma no isan (Devil's Heritage, 1953), a documentary novel, is an account of the bombing of Hiroshima through the eyes of a young Tokyo reporter, handling, among other topics, the death of his Hiroshima nephew and survivors' reactions to the Atomic bomb Casualty Commission, the U.S. agency that conducted research on atomic victims.

Agawa's four major biographical novels are Yamamoto Isoroku (山本五十六, 1965), Yonai Mitsumasa (米内光政, 1978), Inoue Seibi (井上成美, 1986), and Shiga Naoya (志賀直哉, 1994).