Full Idea
Soldiers fighting for an aggressor state are not themselves criminals: hence their war rights are the same as those of their opponents.
Gist of Idea
Even aggressor soldiers are not criminals, so they have equal rights with their opponents
Source
Michael Walzer (Just and Unjust Wars [1977], 08)
Book Reference
Walzer,Michael: 'Just and Unjust Wars' [Penguin 1984], p.136
A Reaction
Walzer's main support for this is that opposing armies never regard one another as intrinsically criminal. It seems inevitable, though, that even the invaders themselves see that they are a bit more criminal than the defenders.