There can only be one side that can be acting justly in a war, and as you have pointed out, when does that ever happen? I disagree with you that it must necessarily be the case that both sides must be aggressive, but in the real world, that it how it ends up. Here are some of my thoughts on the matter.
Would you have examples for a side acting “justly” in a war? How I see war is usually the result of conflicts of interest between several parties. When they don’t get settle diplomatically, the guns start talking. So I think there are justification for war possible assuming the principle of souvereignity that may have to be defended or have an obligation to act.
A libertarian solution to military ntervention would be quite easy I think. Those that think military intervention has to take place will simply have to assemble THEIR OWN resources and use that to intervene somewhere.