This Article examines how the principle of reciprocity operates within the international law of war. Tracing the historical development and application of the law, the Article demonstrates that the existing law of war derives from a set of rules that are contingent on reciprocity. Contrary to common understanding, reciprocity strongly influences states’ interpretation and application of the law of war. The Article first identifies an obligational component of reciprocity that restricts operation of the law to contests between parties with parallel legal commitments. Second, the Article identifies an observational component of the principle of reciprocity that permits parties to suspend or terminate observance when confronted with breach. Although the principle of reciprocity was softened by late twentieth century legal instruments, it continues to form a critical component of the law of war and guides both pragmatic and theoretical discourse. Regardless of normative conclusions about reciprocity as a precondition to application of the law, the Article’s reciprocity-cognizant framework for understanding the law of war provides a useful platform for reform efforts.