Well, look at it this way. If God told me to do something, I'd drop everything and do it (after some healthy skepticism, obviously) but if you think being Christian means not doing anything like go into politics or join the army because you think God should come first and you can't serve him properly whilst doing whatever else, I'd have to disagree. God should come first, but our religion basically boils down to "do good, be tolerant and be kind". I don't see how that prevents me from doing anything else.
God is the most important authority figure in my life, but he's also the one that does the least, and by no means is he my only one. Taking a oath of obedience is acceptable, but if God told me personally "Don't do that!" I'd take it as read that I shouldn't, and thus wouldn't. If a priest told me "don't do that!" well, he's a priest, not God. As for considering the religious ramifications of other soldiers, that assumes they're all Christians too. Other religious have other structures and that's fine by me. I won't shove religion down someone's throat if they don't do it to me. Sure, killing is supposed to be a no no, but I always took that to mean murder, unjustifiable or not. A soldiers sins are not as bad as a mass murderers. Nowhere near as bad, even assuming they
a) Look upon them as sins
and
b) Are in fact Christians.