Following the end of the War in the Pacific, Hara was arrested by authorities of the U.S. Navy and then taken back to Japan to be held in the Sugamo Prison in Tokyo because he had been accused of war crimes. Hara was sent to face a military tribunal on the American island of Guam, and there he was convicted along with other Japanese officers of "neglect of duty in connection with violations of the Laws of War committed by members of their command" for allowing the execution of U.S. Navy aircrewmen who had been captured during air raids on Truk. Since he was the commander of the atoll, Vice Admiral Hara was of course the highest-ranking officer from Truk to be tried for war crimes. Hara was convicted, and he was sentenced to six years in prison. He was then sent back to Sugamo Prison to serve his imprisonment.