Judith in London asks a good question concerning the traditional criteria of the just war. Where, she asks, does the "likelihood of success" criterion come from? The traditional criteria - they can be stated and parsed in different ways, of course - are (1) just cause (2) just authority (3) just intent (4) just conduct (5) likelihood of success (6) good outweighing evil consequences. You can state these differently and you can add several more, such as the requirement, popular nowadays in some circles, of a "just peace."
As I've stated them here, however, numbers 5 and 6 are consequences-based criteria. That is, they are concerned not with rights as such, but instead with the consequences that might occur even if one is, indeed, exercising one's moral rights. Christian just war theory has traditionally accepted that the criteria of the just war are "mixed" as between those which are about the enforcement and defense of one's rights and the rights of a political community - just cause, for example - and those which are about taking into account ultimate real world consequences. Sometimes the consequentialist criteria are referred to as "prudential," although that has seemed to me to underplay that they are indeed moral criteria, albeit of a different sort.
Likelihood of success matters, on traditional just war theory, because, it is said, if you can predict reliably in advance that your fight is suicidally destined to lose, then you should not engage in the evils that will inevitably result from the fact of fighting alone. The fight might be a just attempt to vindicate your moral and political and legal rights - but if it is obviously doomed from the start, then you should avoid it even at the price of injustice.
Judith asks why Michael Walzer (photo above) does not give an account of this in Just and Unjust Wars. In part the answer is that the book is not really concerned with accounting for the traditional criteria of the just war at all; it is a narrower theory than that (in a certain sense, and a wider one than just war theory, in another), a theory of resistance to aggression. More importantly, however, Walzer does not accept this traditional Christian constraint on the attempted vindication of rights. Walzer might say that this is because, as a factual matter, apparently lost causes turn out not to be lost after all - who would have thought, for example, that Churchill would come back to win and, for that matter, Europe was quite convinced that the North could never win the civil war and thought it a matter of moral obligation to intervene to end the killing.
But in fact Walzer means something much stronger in moral rights terms - his argument is not factual in principle, but a matter of basic moral principle. He says, at the opening of chapter 4, p. 51:
"The wrong the aggressor commits is to force men and women to risk their
lives for the sake of their rights .... Groups of citizens respond in different
ways to that choice, sometimes surrendering, sometimes fighting, depending on
the moral and material conditions of their state and army. But they are always justified in fighting; and in most cases, given that harsh choice, fighting is the preferred moral outcome." (italics added)
Walzer grants that citizens may decide not to fight, and likelihood of lack of success may be their motivating reason, but he reserves to them the right to fight absolutely and lauds the preference to fight no matter what. Indeed, he goes on to say that it is the "justification and the [moral] preference" for fighting that "account for the most remarkable features of the concept of aggression." (p. 51) (It is hard to believe that Walzer was not thinking in this passage of the failure of the French to fight in the Battle of France.)
Given this kind of language, it seems evident that Walzer does not regard the criterion of likelihood of success - a consequentialist criterion - as having the power to modify how people should act to vindicate rights in the way that the medieval Christian theologians did. Given that Christian theology is not exactly warm to consequentialist arguments much of the time, what might account for the fact that Christian theology accepts it here as against rights-language more than Walzer will allow?
The deepest reason, I believe, has to do with the moral viewpoint from which one views war - the moral place on which one stands to look at war. The Christian fathers, by the nature of their moral place, inevitably see war from the standpoint of God in Heaven, from above, and this "outside" of humanity standpoint makes it much easier for them - altogether curiously, I emphasize - to consider the costs and consequences of war without regard to rights, given that so many innocents in war suffer. It is easy to imagine the angels in heaven simply writing off the right and wrongs of war and instead seeing only the costs of war - calling for an end to the bloodshed as being the most important thing, even if there are indeed rights and wrongs - they, after all, will get sorted out later, in the final judgment of souls. (I am reminded just a little of the great Swiss playwright Friedrich Durrenmatt's 1950s play, An Angel Comes to Babylon.) Unsurprisingly, this is how Christian theologians of mainstream persuasion tend to see war today - they tend to leave aside the actual reasons for war in order to focus on the costs to innocents and hence rarely find war justified, finding nealry always that it fails that other test of consequences, greater good than evil.
Walzer, by contrast, is in his moral "location," if I can describe it as such, much more like the great secular moralistes, Albert Camus above all, and Rene Char as well, who refuse to depart, as it were, to the heavens, but remain on earth and locate themselves morally in the struggle between right and wrong, good and evil. In that case, the consequentialist considerations about war, so easily visible from the remoteness of heaven, seem much less important than the nearby, concrete causes of war here on earth.
Yet we want it both ways - and this is true of Walzer as it is true of the ancient theologians. The differences are in degree - profound differences of degree - but certainly Walzer will not say that consequences are irrelevant, nor do the ancient theologians write off the discourse of rights which is, after all, the natural language of natural law. Far from it; all hands admit it is a mixed theory, seeking to vindicate plural moral goods.