This is one question I haven't explored much with anarchists and strikes my curiosity. Did any of the heavy weight ancaps like Rothbard or Hoppe write about their idea of just war?
When you think about wars, historically, and the behavior of generals and leaders, the ones who win the wars were always willing to sacrifice the neutrality of third parties, conscript soldiers, permit collateral damage, and so on, in the service of the greater strategic necessity.
I am pretty much totally unfamiliar with how ancaps or voluntarists would approach the question of war. I know they believe in self defense, but that doesn't explain how they would respond to contingencies in which successful self defense, strategically, required things like seizing defensible area, resource depots, etc.