Moral distance
Our cause is always just and legitimate. The enemy is a lawbreaker, a tyrant, a criminal, a villain without honor and morality. He attacked first and murders innocent babies in their cradles as well. We condemn his actions, we have to punish him and restore the lawful order, peace and freedom. Familiar?
Useful tool in civil wars or wars between culturally very similar sides. Or when pulling the racial card isn't too practical like in Vietnam where they apostrophed the war as a crusade against communism.
...there is a danger associated with moral distance. That danger is, of course, that every nation seems to think that God is on its side.
Police forces has the same mentality btw.