I think most just war theorists would be inclined to agree that war is not a special ethical domain, and just war theory is just general ethical/political theory applied. That point is sometimes lost in discussions of just war theory today (at least popular ones--it is commonly brought up, to be rejected for a more comprehensive form of non-violence, in lefty Catholic circles). People will occasionally suggest that just war theory is "obsolete" these days, because modern warfare is especially violent and the criteria for a just war are never met. I think there are reasons to be skeptical of that claim, but in any case, even if it's true, it doesn't show that just war theory is obsolete and ought to be rejected; it just shows that just war theory says that no wars today are just. There's this idea that the point of just war theory is to justify war, so if it can't justify war, it has failed.
That is to say: the question of whether it's true, and whether there are arguments for it, is left out of account. In my view, just war theory must reduce to to some general ethical theory, if it's to be defended. I cannot see why, otherwise, anyone should accept it.
(My sense, further, is that just war theory should be formulated so as to make claims about what individuals are permitted to do--whoever has the power in a state to go to or conduct war.)