He went to England in June 1941, where he was attached to the American Embassy in London as Assistant Naval Attaché. In September of the same year, he assumed duties as Special Naval Observer and additional duty on the staff of the Chief, Combined Operations (British) as an instructor in amphibious warfare, and was appointed to the rank of colonel in December 1941. In July 1942, Hart participated in the Dieppe Operation, for which he was specially commended for outstanding conduct by Lord Louis Mountbatten, Chief of Combined Operations. He remained in England until October 1942, when he returned to the United States to become a member of the Staff of the Commander-in-chief, U.S. Fleet, Navy Department, Washington, D.C., as Chief, Future Plans Section.