 Even supposing that a war has been undertaken rightly, it must also be fought rightly for it to be just. And here we come to ius in bello, a topic treated in DIB III. Grotius begins by setting three rules governing the conduct of war (DIB III.1.2–4), the first and most basic of which is that “In war things which are necessary to attain the end in view are permissible” (III.1.2). This obviously places wide limits on permissible conduct, though it isn’t as chilling as a first reading might suggest, for the necessity requirement can be hard to meet. After advancing these rules, Grotius considers the permissibility of a large number of actions. The variety and amount of detail in this discussion is fascinating. Typical is his analysis of ruses, deceit and falsehood. Not only does he distinguish between those three ways of conveying false impressions but also he distinguishes variations within each of them (see III.1.6–20). For example, if deceit is a genus, then deceit in a negative action is one species (III.1.7) and deceit in a positive action is another (III.1.8). And deceit in a positive action can in turn be divided into two sub-species (ibid.). The practical aims of DIB come through plainly here and throughout Book III, where the priority seems to be clarifying what exactly is and is not permissible in war by considering a large number of actions which could belligerents could undertake and determining their moral and legal standing. In general, Grotius held that war is justifiable when, and only when, it serves right. Since the conditions for service to right are numerous and non-obvious, he must expend considerable effort identifying and explicating them (for more, see Draper 1990: 191–207).