Chapter 1 focuses on the early modern theologians and jurists of Spanish scholasticism. It begins by discussing the theological and metaphysical premises on which they built their doctrine of just war, and then turns to examine how they used the vocabulary of the Roman ius gentium in their construction. The chapter underscores the theologians’ self-understanding as providers of guidance for priests administering the sacrament of confession. The law of nations was central to their solution to the questions of whether and how war could be fought without sinning. Rulers who waged war to enforce their rights were free of sin, as were also soldiers who followed the command of legitimate rulers. While this solution worked for confession, it created a juridical problem the scholastics recognized but did not solve: rulers are judge and party in disputes that lead to war.