Francisco de Vitoria’s ius in bello

Francisco de Vitoria (1483/1486 – 1546), a Spanish Dominican friar and the dean of the School of Salamanca, developed a comprehensive theory of just war. In this theory, he addressed ius ad bellum, ius in bello, and ius post bellum. His interpretations of ius in bello – the rules for waging a just war by a just side against an unjust side – are particularly useful, even to this day. This paper briefly analyzes these rules and the solid, yet heterogeneous arguments Vitoria used to prove their validity and correctness. Vitoria’s ius in bello corresponds to his understanding of a just war as the judgment of an unjust side by a just side for harm caused. Thus, it primarily regulates the conduct of the just side in an ongoing just war, giving it superiority over the unjust side. Conversely, it restricts the just side’s use of military force against the unjust side, a concept that is a precursor to the modern law of war.