HUGO GROTIUS (1583–1645) has long been regarded as a founding figure of modern international law. In particular he is credited with the secularisation of the natural-law approach: ‘Grotius finally excised theology from international law,’ writes Shaw, ‘and emphasised the irrelevance in such a study of any conception of a divine law.’ More recently he has been studied as a seminal political theorist of the early modern period who began the investigation of violence and accountability that was to be continued by Thomas Hobbes and John Locke: ‘seventeenth-century social contract theory is better seen as a conversation among Grotian thinkers than as a quarrel between “Leviathan” and constitutionalism’, suggests Baumgold. A consequence of such views of Grotius's contribution to international law and political theory is that the role that Christianity plays in his outlook is downplayed. This chapter has two aims. First, it sets out the ideal as regards the treatment of civilians in war as enunciated by Grotius in the early years of the Thirty Years War. Secondly, this chapter pays attention to the role that Christianity plays in Grotius's endeavour to limit the harm of war through the construction of a multi-faceted pro-peace ideology.