To expound the law relating to war was a primary purpose of Hugo Grotius in the writing of his famous treatise, De jure belli ac pacis (1625). In Grotius’ opinion, a ‘very serious error’ had taken hold of the popular mind, to the effect that there was no law regulating the manner in which the combatants went about their deadly business. The events of the Thirty Years War, raging in central Europe at the time the book was written, could easily have given rise to such a notion. Be that as it may, one of Grotius’ central concerns was to refute this pernicious misconception. Even in time of war, he insisted, the opposing sides remain part of a common moral community, governed by the general law of nature, and also by the body of customary and contractual law known as the law of nations.