Thanks for the question.
Let's simplify things a little.
The basic rule is you can never do evil, whether under orders or not. Defending one's country by engaging in acts of war is permissible when there is a just war. When serving an unjust government, aggression is not permitted, hence the Germans who served Hitler have no excuse. Your responsibility in the military is to obey orders, unless there is clear evidence that it would be in contradiction to God's law, and thereby evil. So, even the Germans should have told their commanding officers "no" and taken the punishment. To do otherwise would be to commit grave sin. If a person fails to see the evil because of their limited understanding of events, they can seek out God's forgiveness, but then they should refuse to participate in the evil.
With the current leadership our country has, I would not judge anyone who has doubts about their leader's capacity to act justly and prudently. Making a decision to serve one's country, however, often transcends current administrations and could have lifelong commitments. God's aid should be implored when making such an important decision.
If you are a person of good character, and want to do God's will, then you are the kind of person we need in the military. We don't need persons who don't care and blindly follow orders without ever reflecting on the rightness of the actions.