According to the traditional theory of just war we have got two sets of principles, ius ad bellum (that is, that govern the resort to and continuation of war) and ius in bello (that govern the conduct of war). The theorist of the traditional theory of just war, Michael Walzer, claims that those principles are logically independent, and the foundation of this claim is that it makes no difference to the permissibility of an unjust combatant’s conduct in war. Walzer implies that just and unjust combatant -in appeal to Doctrine of Moral Equality of Combatants which includes ius ad bellum – has “an equal right to kill”. This means that unjust combatants do not do wrong by participating in the unjust war (but they do wrong only if they violate the principles of ius in bello). Thus they are not morally responsible for participation in the unjust war.
Jeff McMahan polemicises with Walzer. He thinks that those principles are not independent. Unjust combatants cannot join the war and participate in war without doing wrong, thus they do not obey ius ad bellum principles and, automatically, they cannot comply with ius in bello principles. McMahan undermines the Doctrine of Moral Equality of Combatants. According to McMahan’s assumptions, what will the moral responsibility of unjust soldiers look like? To answer this question Jeff McMahan challenges Walzer’s arguments: The Argument from Coercion (The Gladiatorial Combat Model of War); The Argument from Institutional Commitment which includes The Subjective Justification - The Epistemological Argument (in McMahan’s term).