The doctrine that a state may justly go to war for some restricted reasons, which are centrally those of self-defence, and the rescue of another state from an aggressor. Problems include deciding whether self-defence may be broadened from defence against actual attack to defence against threats, or against perceived threats, and whether it is permissible to make pre-emptive strikes. In addition to theorizing about when it is just to go to war (jus ad bellum), just war theory embraces principles about the way war may be conducted (jus in bello), generally ruling out gratuitous violence, assassinations, war against civilians, and so on, although grave difficulties confront any distinction between those parts of a population engaging in modern war, and those innocents whose work is irrelevant to it.

http://www.bbc.co.uk/religion/ethics/war/ A survey of related philosophical, religious, and legal debates