Full Idea
When a country's right to war is questionable and uncertain, the constraints on the means it can use are all the more severe.
Gist of Idea
If an aggression is unjust, the constraints on how it is fought are much stricter
Source
John Rawls (A Theory of Justice [1972], p.379), quoted by Michael Walzer - Just and Unjust Wars 14
Book Reference
Walzer,Michael: 'Just and Unjust Wars' [Penguin 1984], p.229
A Reaction
This is Rawls opposing the idea that combatants are moral equals. The restraints are, of course, moral. In practice aggressors are usually the worst behaved.