The retreat of the Byzantine Army from Syria in around 650 CE, in advance of the approaching Arab armies, is one that has resounded emphatically in the works of both Islamic and Christian writers, and created an enduring motif: that of the Islamic-Byzantine frontier. For centuries, Byzantine and Islamic scholars have evocatively sketched a contested border: the annual raids between the two, the line of fortified fortresses defending Islamic lands, the no-man's land in between and the birth of jihad. In their early representations of a Muslim-Christian encounter, accounts of the Islamic-Byzantine frontier are charged with significance for a future 'clash of civilizations' that often envisions a polarised world. A. Asa Eger examines the two aspects of this frontier: its ideological and physical ones. By uniting an exploration of both the real and material frontier and its more ideological military and religious implications, he offers a more complex vision of this dividing line than has been traditionally disseminated. With analysis grounded in archaeological evidence as well the relevant historical and religious texts, Eger brings together a nuanced exploration of this vital element of medieval history.