Col (Dr) Don Snider (USA, Ret) has a well thought-out article on the Christian military witness at Journal of Faith and War.  He notes the complexity of the issue with a single opening phrase:

There are no formulas, Christ only said: “Render unto Caesar what is Caesar’s, and unto God what is God’s.”

His points fall under the following categories:

Mastering the Context
“…you are not to compartment your life and its varied roles (supposing that you actually could do so psychologically which is, of course, quite debatable). You may not choose to be a Soldier leader in some settings and a Christian in others.” 

“You are, in addition, free to witness appropriately to your faith, Read more