In February 1942, after the United States' entry into World War II, Zemke desired to join a United States Army Air Forces (USAAF) unit and made his way back to the United States through Iran and Egypt. After several temporary assignments, including tests of the new P-47 Thunderbolt, Zemke, by then a major, became group commander of the 56th Fighter Group—the first fighter group to fly the P-47—on September 16, 1942, preparing it for movement to England. Not impressed with the performance or maneuverability of the aircraft, Zemke spent much time developing tactics utilizing the Thunderbolt's tremendous dive capability that would make it a successful platform in the European Theater. In January 1943, the 56th was sent to England in order to participate in the bombing offensive against Germany with the Eighth Air Force.
