Why think that certain kinds of behaviour can make us liable to harm?
Just causes involve “the prevention or correction of wrongs that are serious enough to make the perpetrators liable to be killed or maimed” McMahan, “Just Causes for War”.
Why think that all combatants have the right to attack combatants on the other side, regardless of whether their war is just?
“our right not to be attacked … is lost by those who bear arms … because they pose a danger to other people” Michael Walzer.
“Allied soldiers landing on the shores of Normandy during World War II were fighting against genocide and imperialist expansion; their adversaries were defending those iniquitous ends.
Why should the Allies lose their rights, only by doing what they are clearly morally permitted, perhaps even required, to do?” Seth Lazar “Evaluating the Revisionist Critique of Just War Theory”