That war is sometimes necessary in a fallen world – see J Edwards earlier response.
That the government has been given the sword and the authority to use it, and that we must submit to the governing authorities for the sake of righteousness – See Romans 13:1-7. While not stated explicitly, the principle in verse 7 would cover any lawful duty to the government, including obedience in military service.
For the right of self defense, see Luke 22:36. “…and he that hath no sword, let him sell his garment, and buy one.”
Defend the weak. Psalm 82:2-4
Since the Bible cannot be divided against itself, the principles of our behavior (love) toward our enemies and “the evil person” stated in Matthew 5:39-48, must be understood in light of these other principles of scripture, a few of which are provided above, although not a comprehensive treatment of the subject by any means.
While the command to love our enemies, turn the other cheek, etc, does not apply to warring armies per se, there would still be wartime applications such as treatment of prisoners, caring for the wounded, etc. I believe it is possible for a soldier to do his/her duty on the battlefied while not falling into sins of vengeance, hatred, or cruelty.
The answers to your questions, I believe, are implied by scripture, if not explicitly stated (if it walks like a duck and quacks like a duck, it is probably a duck!). By applying these principles of scripture, some of which are provided below, it is possible to arrive at a biblical ethic pertaining to war.
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