The answer is determined by the circumstances. I was in the Military for 5 1/2 years. I was quite incompatible in a number of ways: immature, sensitive, critical thinker, lonely, unfulfilled, and so forth. I was kept constantly angry. I learned to hate. My emotional, mental, and spiritual growth were stymied. At the lower ranks, my superiors were often abusive, crude, sadistic, and disfunctional. In the military one cannot find who they are naturally. I always thought the slogan "We train men.." was quite appropo, especially since "train" is a word we use with animals.
As to actual warfare, one should know their parameters. If you are a Christian, you live in enemy territory alllll the time, and we are here temporarily anyway. Death is not a problem, since "to be absent from body is to be with the Lord. Jesus did not speak against the Roman army or Cornelious for being in the military. I am sure he knew the circumstances would vary widely. HOWEVER, WE KNOW as Christians what we are to do, being busy in our spiritual gifts in the Body Of Christ. Jesus as a warning said, "...live by the sword...die by the sword." There will always be war for those that like war--Jesus speaks of no more war as a future hope--so why jump in the bull ring, in Satan's world.
Free In Christ Jesus our Lord & Savior, as the Father would have it.