Pirenne, Henri, pee REHN, ahn REE (1862-1935), was a Belgian historian. He is best known for his seven-volume History of Belgium (1900-1932), considered the standard work on the subject. Pirenne specialized in studying medieval economic history, and especially medieval cities. Pirenne's works are noted for their scholarship and bold interpretations, often challenging traditional views. His Medieval Cities (1925) and Mohammed and Charlemagne (published in 1937, after his death) became classics of historical interpretation, and are still widely read. Pirenne theorized in Mohammed and Charlemagne that the Muslim conquests, rather than Germanic invasions, ended the Roman Empire and the ancient world. But many scholars now dispute his theory.