Armies, merchants, travelers, pirates, captives and slaves crossed back and forth across the constantly shifting, invisible line that separated the Ottoman Empire from the darulharb, the "Abode of War." In the binary world of Islamic law, divided into darulislam, the "Abode of Islam," and darulharb, it fell to the Ottomans' chief Islamic religious authorities to rule on the pressing questions that concerned those who passed between the two realms. Though true to the tenets of Hanafi jurisprudence, the legal responsa (fetva) of the Ottoman Grand Muftis, the seyhulislams, were issued in response to real queries and reflected contemporary religious, political, military, and economic concerns. As the borders of the empire expanded and contracted over the course of the early modern period, the nature of the questions changed and developed, and so too did the rulings of the seyhulislams. Utilizing seyhlislam fetva collections from the sixteenth through the eighteenth centuries, this paper explores the changing ways in which the Ottoman religious authorities confronted the questions concerning captivity, conversion, and apostasy that stemmed from the endless stream of foreigners, subjects, and slaves crossing, not always willingly, between the worlds of war and peace. Indeed, the fetvas have much to tell us about the experiences of prisoners-of-war, victims of pirates, slaves, apostates, protected foreign merchants, and so forth. Moreover, because the fetvas in each compilation represent only a tiny fraction of an individual seyhulislam's total output, the selections chosen by the compilers indicate the kinds of questions that were most pertinent at the time. Though fetvas were non-binding and theoretically did not create precedents, fetva collections were frequently referred to by provincial judges (kadis) for guidance and therefore had a significant impact on how similar cases were handled in courts throughout the empire. Thus, the interpretation of Islamic law shaped and was shaped by the policies of government, changing frontier realities, and the experiences of those individuals who crossed the line.