ABSTRACT
The Decretum emerged out of a European society that, from the turn of the second millennium ad, was increasingly legalistic and sensitive to the possession and protection of rights, recognizing a strong relationship between right (ius) and power (potestas). By promoting the concept that war in defense of the Church was not only free from sin but also potentially spiritually meritorious, Gregory VII opened the door to later theories of just war, holy war and crusade. Gratian's Decretum was forged in the crucible of this reformatory period and was perhaps its most important product, marking the beginning of classical canon law and just war doctrine proper. This chapter claims that Gratian is the single most influential figure in the history of the Western just war tradition. This is largely a result of the immense popularity and influence of the Decretum during the medieval and early modern periods.
