‘What is Caesar’s?’: Canon Law and the Constraint of the Prince in Medieval International Law

Abstract This article analyses the influence of developments in canon law on European international law between the 12th and 16th centuries. It discusses the construction of sovereignty; the role of canonical civil procedure in regulating disputes between princes; and efforts by canonists to limit the destruction caused by medieval warfare. Throughout, historical examples are provided to illustrate the effects of these developments in practice. The article concludes that medieval canon law had the effect of constraining sovereign power and limiting the prince’s ability to act with impunity.