I don't think it is. The Lord sure did send a lot of people into battle in the OT, but for just reasons. The gray area is motive for the fight I think. I have a sneaking suspicion that the Lord never sent His people into battle over oil or poppy fields, lol.
Nowadays, the military can create a dilemma for the Christian recruit. From what I understand, most military personell don't know where they're going or why until the get on the plane. In this case, I personally believe that...they signed up and should obey orders. The Lord would know what's in their heart while they're active duty and probably hold the higher ups responsible in the long run, those who gave the orders... *Probably*
I do know that the Lord wants us to have honor and fulfill our earthly contracts that we make.