Are you allowed to serve as a Christian in the army?

Luther:

Good question! Because you can only fight well and bravely when you have a good conscience. Everything works better then. But when you suffer from your conscience, then your heart cannot be brave, but you become weak and afraid.

It looks like it is not something done, that a Christian fights and kills. But it is like it is with a good doctor: When a disease is very dangerous, he has to amputate a hand, foot, ear or eye to save the rest of the body. When you look at that one body part, then it seems as if the doctor is a bad person. But when you look at the whole body, then you see that he still is a loyal and good person. So it is with war. When a soldier protects honest persons, wife and child, hearth and home, then it is a godly work.