Mutual aggression doesn't work at all from a legal perspective. If two people try to settle a dispute with a duel and both agree, and one dies. The other is going to go to jail. On the other hand if someone in the military kills an innocent person nothing happens to them. Under the law you can't fight in duels but you can blow up a building and kill any innocent people who happen to be standing next to it.

It does kind of work on a moral ground though. People often object to wars because of the innocent people that will die, and even though its illegal most people are not outraged by two consenting adults fighting to death.