By 1951, Hilsman had risen to the rank of major. He worked on planning for the North Atlantic Treaty Organization and of Supreme Headquarters Allied Powers Europe with the Joint American Military Advance Group in London in 1950 to 1952 and as part of the International Policies Division of the United States European Command in Frankfurt, Germany, in 1952 to 1953. Waiting for the end of hostilities in the Korean War, he resigned from the Army in 1953 but kept reserve status.