Full Idea
Justice of war [ad bellum] and justice in war [in bello] are logically independent. It is perfectly possible for a just war to be fought unjustly, and for an unjust war to be fought in strict accordance with the rules.
Gist of Idea
Jus ad bellum and Jus in bello are independent; unjust wars can be fought in a just way
Source
Michael Walzer (Just and Unjust Wars [1977], 02)
Book Reference
Walzer,Michael: 'Just and Unjust Wars' [Penguin 1984], p.21
A Reaction
The perfect decorum of an unjust firing squad might even make the crime worse. There is something chilling about an evil army conducting itself perfectly within the ethics of warfare. Better than the other thing, though. McMahan disagrees.