My friend Gabriel asked me to detail the arguments for "Just War". So, that's what I'm going to attempt to do here.
The underlying assumption is that people are broken (fallen, depraved, whatever you want to call it -- the Doctrine of Original Sin is the one doctrine that appears to be emprically verifiable from even a cursory examination of human history). There is something unmistakably wrong with us, but we are not savages. We have it in us to live in civilized ways, establishing rules to live by and actually restricting our personal freedoms on occasion for the benefit of others.
Sometimes, however, people choose to use their personal freedom to exploit others and rule over them via domination. What are we to do when we see people being cruelly oppressed? Is it ever appropriate to take up arms in defense of helpless people?
Centuries ago, St. Augustine looked at the world and at the Bible and said, "It is sometimes appropriate to do that, but we need rules about when to take up arms and how to do it."
He established a set of rules -- rules of engagement -- that have been tweaked and modified over the years. I will do my best to summarize these rules of engagement here. I know there is some disagreement over some of these, but this is just an overview.
First, there must be a "just" cause. There must be a good reason that is clear and conscionable (to stop genocide, human rights violations, the immoral treatment of large numbers of people or an invading force, etc.).
Second, there must be a "just" authority who will announce that there is going to be a war. This must be a recognized and legitimate authority -- not a faction or a group of radicals. It must be appropriate, internal leaders who follow some form of a constitutional law. Since World War II, it has been recognized that the international community should be involved on some level.
Third, it should only be viewed as a last resort. All means of non-violent action must be exhausted (discussion, diplomacy, economic sanctions, boycotts, etc.).
Fourth, it must be entered into with "just" intentions. A war may only be waged if its outcome imagines a secure peace for everyone involved. The motives must not be any kind of imperial conquest or economic gain.
Fifth, there must be a probability of success. We must not send people to fight a battle that cannot be won. I think this is the point at which many people might speak up in regards to our current war.
Sixth, there must be proportionality of cost. In other words, the total good achieved by a war must outweigh the evil and suffering that the war (or absence of war) would cause. No war should produce a cure that is worse than the disease.
Seventh, there must be a clear announcement. Those declaring war must announce their intentions, AND they must announce what could be done to avoid the conflict. There should be no surprise attacks like at Pearl Harbor. Acts of terrorism are not preceded by clear announcements of either intention or what could be done to avoid them. Therefore, terrorism is not just.
Finally, the war must be waged with "just" means. According to most advocates of Just War, the use of weapons of mass destruction would be wrong because they tend to create too much collateral damage. No chemical weapons allowed. No bombing of villages. No killing civilians. No torture.
Just War advocates believe that under certain circumstances, it could be a God-honoring thing to use force in order to secure a better world -- especially in defense of marginalized people groups.
What about you? What do you think? Do you think it's ever acceptable to take up arms and kill someone in a war? Could you do it? What about if someone broke into your home and threatened your loved ones? Is it okay to use violent force then?
These are not easy questions, and they have no easy answers. Please don't insult anyone's intelligence here by pretending they are. Respond thoughtfully, please.