Military service

i) On the face of it, military service seems to be morally problematic. Take volunteers. When you enlist, you sign a blank check. You don't know in advance what the foreign policy will be from one year to the next. You may disapprove of what you're ordered to do. Is it ethical to make an open-ended commitment like that? Especially given the moral stakes of warfare? The snap judgments. And so on. 

ii) However, that must be counterbalanced by another consideration. The justification for national defense is that national defense is a logical and necessary extension of self-defense. Sometimes we lack the individual resources to protect ourselves or our dependents. Say, if we're outnumbered or outgunned. Therefore, self-defense may necessitate pooling our collective resources to mount a common defense. That confers a general benefit.

Assuming that we have a duty to protect our dependents, and in some cases our neighbors, or even strangers, military service is an unavoidable–although not a universal obligation. 

iii) Hence, military service generates moral dilemmas. Conflicting duties or competing goods. Given the dilemma, I think men who find themselves in that predicament are in a situation of diminished moral responsibility. What would ordinarily be wrong is mitigated or exculpated by the moral dilemma. 

That's not carte blanche to follow just any order. It doesn't excuse atrocities. Some actions are intrinsically wrong. But there's greater moral flexibility in that situation than situations where you have one unimpeded duty.