The Boston Review has a long, but thought-provoking article on whether soldiers should be able to opt-out of conflicts they believe are unjust.
The gist of the argument goes like this:
1. Most Wars are Unjust. Both sides of a war can't be just, so every war is either a conflict between a just side and an unjust side or between two unjust sides. So, if you find yourself fighting in a war, statistically you're more likely to be fighting on the unjust side.
2. It's Wrong to Fight for an Unjust Cause. Soldiers ultimately decide whether to participate in a war, and they bear responsibility for that decision. It's not just to attack even enemy soldiers for an unjust cause, just as it would be unjust for a criminal on the run to shoot a police officer.
3. Therefore, soldiers should be able to opt-out of conflicts they believe are unjust. This is just an extension of the current rule that soldiers must disobey orders during combat that they believe are unjust.
Of course, Point Number 1 above isn't really necessary for the conclusion, but it makes the issue more pressing. Otherwise, you could simply say, 'well, this isn't important because most wars are just anyway.'
So, is Point Number 2 right?