After the end of the war, Koehl made a name for himself for his research on national socialism, notably of studies on Heinrich Himmler as Reich Commissioner for the Consolidation of German Nationhood, as well as a pioneering work on the Nazi German during the Occupation of Poland (1939–1945). Koehl coined the term "neofeudalism" for the characterization of the national socialist rule, and completed an account of the history of the SS in 1983.