From Wiki ;
In 1940, Zemke was sent to England as a combat observer with the Royal Air Force, studying the tactics of both the RAF and the Luftwaffe, observations that he would use later when the United States entered the war. In 1941, he was sent to the Soviet Union to instruct Russian pilots in flying lend-lease P-40 Tomahawks.
World War II service
56th Fighter Group
In February 1942, after the United States' entry into World War II, Zemke desired to join a 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.
I had heard that Zemke made his own way back to England without orders so that he could use his knowledge and experience to shape the US squadrons being brought up to operational readiness. I believe that this irked his superior officers who held his promotions back for the remainder of his career. I can not validate this - but believe it may well be the reason he never got beyond Colonel.