Just War Conundrum
According to traditional just war theory, one of the prerequisites for a war's being just is that it have a legitimate aim; that's not sufficient, but it is necessary.
But what about cases where the decision to go to war has several aims, some just and some unjust? For example, in the war of 1812 the United States arguably had both a just aim (resisting British impressments of American citizens at sea) and an unjust aim (a land grab in Canada); likewise in the U.S. Civil War the Confederacy had both a just aim (resisting tariffs and centralised power) and an unjust aim (preserving and extending slavery).
I presume that just war theorists have addressed this question, but I couldn't find a discussion of it in my (admittedly cursory) online search; but here's my suggestion as to how this problem should be handled.
What we need to know, in order to apply the just cause criterion, is how central the unjust aim is to the war's objectives. It seems to me that there are four possibilities, depending on the necessity and/or sufficiency of the unjust aim. (I should add that when I talk of necessity and sufficiency here I am talking about the logical necessity or sufficiency of an aim for the action of which it is a part, not the causal necessity or sufficiency of an aim for a temporally subsequent action; I am thus not assuming any sort of causal determinism. In Kantian terms, I'm talking about what's part of the maxim.)
- First possibility: The unjust aim is both necessary and sufficient for the decision; that is, the decision would not have been made if the unjust aim were absent, and it would still have been made if the just aim had been absent (which implies that the just aim is neither necessary nor sufficient).
- Second possibility: The unjust aim is necessary but insufficient for the decision (which implies that the just aim is likewise necessary but not sufficient).
- Third possibility: The unjust aim is unnecessary but sufficient for the decision (which implies that the just aim is likewise unnecessary but sufficient).
- Fourth possibility: The unjust aim is both unnecessary and insufficient for the decision (which implies that the just aim is both necessary and sufficient).
I think the first and second possibilities are clearly cases where the war fails the just cause requirement. If the war wouldn’t have occurred without the unjust motive, mustn't the war be unjust? The third possibility is trickier, since in this case the unjust motive is superfluous; but in this case the just motive is superfluous too. The decision to wage war thus expresses a kind of indifference to the war's justice which for me inclines the balance toward saying it’s unjust. In the case of the fourth possibility, however, the unjust motive really is merely along for the ride while the just motive is doing all the work, so I'd say in this case the war passes the just cause criterion.