To recapitulate, we have seen that the ad bellum requirements of proportionality and reasonable chance of success sometimes yield the conclusion that a belligerent must prematurely end a war which it was all-things considered justified to wage in the first instance. So does the requirement of discrimination, which insists that combatants not target innocent civilians. Suppose, for example, that A justly starts a war at t1 and can win its war by targeting only military objectives. At t2, however, the enemy has procured weapons which A’s forces cannot counter, and the only way for A to win its war is by deliberately targeting civilian populations. Under those circumstances, A is under aprima facie obligation to stop.
The obligation is only prima facie however because there is one, very restricted case where the deliberate targeting of innocent civilian populations is morally justified: where it is a necessary means to adverting a grave evil such as mass extinction, genocide, and mass enslavement. (Or so I argue in CW, s.7.2.) If targeting would be morally justified at t1 on such grounds, then it is morally justified at t2.