In a series of articles and addresses which culminated in the book Mohammed and Charlemagne, Pirenne advanced the thesis that the Ancient World ended only after the Arab invasions of the seventh and eighth centuries had swept around the perimeter of the Mediterranean and had converted it into a Moslem lake upon which, as one Arab writer graphically said, the Christians could no longer "float a plank."
The waters of the Mediterranean have held the Roman empire like cement; over its waters had passed trade and commerce, the Roman military and naval might, and the vital exchange of ideas. The German tribes that had occupied the western Empire had not, according to Pirenne, destroyed this Mediterranean unity of the Roman Empire. …
