Hugo Grotius is best known as one of the originators of international law. Philosophers of the period also saw him as making a fundamental break with ancient philosophy and Thomistic classical natural law deriving from Aristotle. Grotius carried forward an important distinction made by Francisco Suarez between “law” and “counsel.” Good moral reasons may counsel action without yet requiring it. Law, by contrast, obligates, and obligation is conceptually related to accountability. It concerns what we would be blameworthy for failing to do without excuse. Grotius develops a theory of natural rights and law and sets the philosophical agenda by asking what could ground such a law (Anscombe’s Challenge). His response is human “sociability,” by which he means not simply any desire to affiliate, but the drive to live with others on terms of mutual accountability.

This chapter examines the early emergence of civilian protection norms in medieval Europe and traces their development in the religious and secular strands of just war theory. It argues that the Peace of God played social movement in medieval Europe played a key role in the emergence of the principle of civilian immunity. Second, it shows that the theory set forth in Chapter 2 provides a useful account of how the principle of civilian immunity arose in medieval Europe and why it persisted from the medieval period through the Enlightenment. Throughout, it examines the arguments of key just war theorists and international lawyers, including Augustine, Aquinas, Vitoria, Suarez, Grotius, and Vattel.