Three thought-provoking quotes from Jeff McMahan's article Innocence, Self-Defense and Killing in War:
...it is not obvious why a political leader who orders troops into battle is engaged in causing harm while voters in a democracy who demand that the leader should do so are not; or why drivers who transport arms to the troops count as combatants while the taxpayers who provide the arms by paying for them do not; or why a soldier who is asleep or sitting at a desk well behind the lines can be regarded as threatening or causing harm when a civilian editorialist who stirs support for the war is not...
Persons who join the military are typically aware that this abdication of moral autonomy is a condition of military life; indeed, some join the military in part in order to enjoy the freedom from responsibility. They know, in short, that they are allowing themselves to become instruments of the wills of others. There is, moreover, something else they could know with a little reflection, which is that most wars in which people fight are unjust. This follows from the assumption that a war can be just on at most one side, though it can be unjust on both.
Even if this formal assumption is unwarranted, it does seem true as a contingent fact that very few wars, if any, have been just on both sides, while, as Anscombe puts it, “human pride, malice and cruelty are so usual that. . . wars have mostly been mere wickedness on both sides.” Putting these two points together, we arrive at the conclusion that, in joining the military, one allows oneself to become an instrument for the violent pursuit of purposes that are more than likely to be unjust. How can this possibly be a morally acceptable thing to do? Of course, in many cases, the pressure to join the military may be nearly as strong as the pressure, once one is in the military, to surrender the prerogative of determining for oneself whether or not the war in which one is asked to fight is just. It is only when this is true that there can be a convincing case for regarding an Unjust Combatant as morally innocent. For, otherwise, following one’s superiors into an unjust war is roughly analogous to committing a crime while drunk: one may not be responsible for one’s action given one’s condition at the time, but one’s conduct nevertheless remains culpable because of one’s responsibility for getting oneself into a condition of diminished responsibility.
First, the morality of war, and not the rules of war, is what should govern the conscience of the individual soldier. In particular, if the individual soldier has reason to believe or suspect that his country’s war is unjust, this is equivalent to believing or suspecting that his action as a belligerent in this war is or would be murderous. If he is convinced that the war is unjust, then he must not participate.