The implications of the distinction are that jus in bello has to be completely distinguished from jus ad bellum, and must be respected independently of any argument concerning the latter. This is so because ‘the two sorts of judgement are logically independent. It is perfectly possible for a just war to be fought unjustly and for an unjust war to be fought in strict accordance with the rules’. In other words, ‘the limitation on jus ad bellum has no influence on jus in bello’. This is so even though the two bodies of law operate simultaneously in many situations. For, although the mainstream view is that the two bodies of law apply at different stages of a conflict (jus ad bellum affects the legality of the initial recourse to force, whereas jus in bello logically applies after hostilities have begun), it is questionable that this sequential distinction is still relevant. Recent developments have entailed that the two bodies of law no longer operate at different stages; once hostilities begin it is necessary to consider and apply both. jus ad bellum thus applies ‘not only to the act of commencing hostilities’ but also to each subsequent act involving the use of force, which has to be justified by reference to the principles of necessity and proportionality. Simultaneous application of jus ad bellum and jus in bello should not imply that the two concepts are linked or interdependent. Acts that are in complete conformity with jus in bello may nonetheless be prohibited under jus ad bellum. Similarly, an attack that is inconsistent with jus in bello does not necessarily affect the legality of the use of force.

Jasmine Moussa, “Can jus ad bellum override jus in bello? Reaffirming the separation of the two bodies of law,” International review of the Red Cross, vol. 90, Issue 872, 2008, pp. 967-978.