This thesis stood in sharp contrast to that being promoted by another rising young scholar, Gerd Tellenbach, who believed that the nobility of France and Germany owed their origins to Frankish aristocrats placed in high positions over regions conquered by the Carolingians in the eighth and ninth centuries.
This ethno-cultural view of the history and formation of legal and political institutions was strongly represented among a number of nationalistically-oriented German and Austrian medievalists of Schlesinger's generation, including Karl Bosl, Theodor Mayer, and Otto Brunner.
In a famous lecture delivered in 1963, Schlesinger sharply criticized the politically-charged field of Ostforschung ("East[European] Studies"), which had for a long time, but particularly during the Third Reich, served as a thinly-veiled effort to lend scientific credibility to anti-Slavic prejudice and German domination of Poland and other parts of eastern Europe.
Accordingly, Schlesinger wrote extensively on settlement along the German-Slavic frontiers in the Middle Ages, as well as on the development of bishoprics and towns in the Saxon and Slavic areas of eastern Germany, paying particular attention to local and regional contexts for economic and demographic change.
He made seminal contributions early on as well to the important Repertorium der Deutschen Königspfalzen project, which assembled detailed archaeological and historical studies of the sites which had served as royal estates or waystations on the tours of the medieval German kings.