In this contribution to the ongoing debate on the nature and causes of the Islamic conquests in Syria and Iraq during the sixth and seventh centuries, Fred Donner argues for a necessary distinction between the causes of the conquests, the causes of their success, and the causes of the subsequent Arab migrations to the Fertile Crescent. Originally published in 1986. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These paperback editions preserve the original texts of these important books while presenting them in durable paperback editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.