We argue about this a lot. I think clearly the swords were for self-defense, since they were each instructed to "sell their cloak and buy one".
Yet we are also told not to get into fights over unimportant things:
You have heard that it was said, ‘Eye for eye and tooth for tooth.’ But I tell you not to resist an evil person. If someone slaps you on your right cheek, turn to him the other also; if someone wants to sue you and take your tunic, let him have your cloak as well;… Matthew 5:38-40
Even in the ultimate case where the Beast is about to wipe out all Christians, we are told to hide rather than fight. If caught, we are told to die rather than deny Jesus.
That, of course, is because this body is expendable and we immediately get issued a better one when Jesus returns.
So a much bigger game is being played here than just "war and peace."
I'd still like to understand exactly when I'm allowed to use my sword. Until I learn better, it will be when there is a need to defend my family.