Writing on behalf of the Movement for the Abolition of War, Bruce Kent says that distinguishing between just and unjust wars 'ought not to be our major preoccupation'. Rather:
What matters most is the effort we make to prevent wars starting by positive action directed to their political and social causes.
He gives as an example of such political and social causes the settlement imposed on Germany after World War I.
Preventing war where it is possible to do so without incurring costs even worse than the costs of war itself is obviously a worthwhile enterprise. However, no one is obliged by Kent's alternative: we don't have to make either war-prevention or distinguishing between just and unjust wars a more major preoccupation vis-à-vis the other. There is good reason not to do this, because war shares with most problematic forms of human behaviour the feature of being governed not only by social causes but also by human choices. As much effort as one makes working on the causes, it will remain possible that other people will make choices - for war, massacre, genocide etc - with which you have to contend. Distinguishing just from unjust wars will continue therefore to be indispensable.
To put the same thing differently: it is not just up to us, whoever 'we' are, to abolish war, as appealing as the vision of a world at peace may be. If 'we' renounce war, while somebody else doesn't, we won't necessarily achieve an abolition of war, we may simply enable war in its unjust forms - by aggressors, those with territorial ambitions, and so forth. Bruce Kent's recommendation is a bit like directing attention away from meliorative social policies to mitigate the effects of poverty because you think it more important to get rid of poverty altogether. One can keep both objectives in view.