Whole books have been written on these subjects, and there are theologians on both sides of the issue. A few basic principles should be considered. War is never an ideal situation. If we lived in a perfect world, there would never be a cause worthy of war. But we live in a fallen world in which sin has divided man from God and people from people.
War is sometimes necessary.
It is noble and right for a nation to defend itself against oppression, injustice, and other advances of evil in our world. The war between Satan and God has spilled over into human time and space. We are involved, whether we want to be or not. In war, the state bears responsibility for the loss of human life. Soldiers on the battlefield are not committing murder.