The distinction between jus ad bellum and jus in bello was drawn in the traditional theory of the just war long before it appeared in modern international law. The theory of the just war is a theory about morality, not law--although most of its earlier proponents understood it as an account of the natural law. In this moral theory, the main reason for the traditional insistence on the separation of jus ad bellum and jus in bello is that the two doctrines must be independent if the orthodox doctrine of the "moral equality of combatants" is to be justified. The moral equality of combatants is the view that all combatants, irrespective of whether they fight in a just or unjust war, have the same rights, immunities, and liabilities. Under this view, a combatant is not guilty of wrongdoing merely by virtue of fighting for an unjust cause, or in an unjust war.
According to the traditional theory of the just war, responsibility for matters of jus ad bellum lies with the sovereign, not his soldiers. In Shakespeare's Henry V, the king, disguised as an ordinary soldier, is conversing with some of his soldiers on the eve of the battle of Agincourt. He remarks: "Methinks I could not die anywhere so contented as in the King's company, his cause being just and his quarrel honorable." One soldier replies: "That's more than we know," whereupon a second says: "Ay, or more than we should seek after; for we know enough if we know we are the King's subjects: if his cause be wrong, our obedience to the King wipes the crime of it out of us." (1)
What I will argue is that the doctrine of the independence of jus in bello from jus ad helium, and its implication that whether the war in which a combatant fights is just or unjust has no bearing on that combatant's rights and liabilities in the conduct of war, have no foundation in the basic, nonconventional morality of war. If it is desirable in law--and I think it is--to have rules for the conduct of war that are neutral between those who fight in a just cause and those who do not, there must be substantial divergence between the basic, nonconventional morality of war and the law of war.
The fundamental reason why, as a matter of basic morality, the principles of jus in hello cannot be independent of those of jus ad helium is that it is simply not morally permissible to fight in a war with an unjust cause. This is so for a number of reasons, one of which is that acts of war that promote an unjust cause cannot be...