Few U.S. policymakers have heard of Fethullah Gülen, perhaps Turkey's most prominent theologian and political thinker. Self-exiled for more than a decade, Gülen lives a reclusive life outside Philadelphia, Pa. Within months, however, he may be as much a household a name in the United States as is Ayatollah Khomeini, a man who was as obscure to most Americans up until his triumphant return to Iran almost 30 years ago. Many academics and journalists embrace Gülen and applaud his stated vision welding Islam with tolerance and a pro-European outlook. Supporters describe him as progressive. In 2003, the University of Texas...