Following upon this, Duby formulated a famous theory about the Crusades: that the tremendous response to the idea of Holy War against the Muslims can be traced to the desire of iuvenes, knights, mostly young and with little prospect of becoming lords, to make their fortunes by venturing abroad and seeking fame in the Levant.

Throughout the 1970s and 80s, French doctoral students investigated their own corners of medieval France, Italy and Spain in a similar way, hoping to compare and contrast their own results with those of Duby's Mâconnais and its thesis about the transformation of European society at the end of the first millennium.

Although he was never formally a student in the circle of scholars around Marc Bloch and Lucien Febvre that came to be known as the Annales School, Duby was in many ways the most visible exponent of the Annaliste tradition, emphasizing the need to place people and their daily lives at the center of historical inquiry.

In books like The Three Orders: Feudal Society Imagined and The Age of Cathedrals, Duby showed how ideals and social reality existed in dynamic relationship to one another.

The book remains a classic of Annales-style historiography, eschewing the "great man" and event-oriented theories of political history in favor of asking questions about the evolution of historical perceptions and ideas over the long term, the longue durée.

Duby also wrote frequently in newspapers and popular journals and was a regular guest on radio and television programs promoting historical awareness and support for the arts and social sciences in France.