The cause of the break with the tradition of antiquity was the rapid and unexpected advance of Islam. The result of this advance was the final separation of East from West, and the end of the Mediterranean unity.

He also became prominent in the nonviolent resistance to the Germans who occupied Belgium in World War I. Pirenne argued that profound social economic cultural and religious movements in the long-term resulted from equally profound underlying causes and this attitude influenced Marc Bloch and the outlook of the French Annales School of social history.