Following the Prague Spring of 1968, during which a nascent socialist reform movement was put down by an invasion of Soviet and other Warsaw Pact military forces, Graus emigrated and sought asylum in West Germany.
Graus's Czech doctoral thesis, published in 1965 as Volk, Herrscher und Heiliger im Reich der Merowinger (People, Ruler and Saint in the Merovingian Kingdom) was a groundbreaking study of how the early medieval hagiographic texts – dismissed by most historians then as pious fictions with little or no historical value – contained important insights on popular religious sentiments and social mentalities.
As with historians of the French Annales School, Marxist questions, as well as his own Jewish heritage and experience in the war, lead Graus to examine underlying assumptions about power, ethnic identity, social status and the marginalization of certain groups in medieval society.
He thus focused on topics such as the Black Death and peasant revolts that were traditionally treated within a broader scheme of political history, but from a perspective that attempted to get at how common medieval people thought about social justice, violence, ethnicity and religion.
Over the course of his career, much of Graus's work, and particularly his interpretation of social and political institutions, sought to offer a counterbalance to the predominant models of what the Germans call Verfassungsgeschichte, represented by historians like Karl Bosl, Walter Schlesinger and Otto Brunner and which had a strongly conservative-nationalist underpinning.