Hokey-koke, this looks like a damn good debate. As a current student of War Studies, this kind of thing makes me more excited than I'd normally like to admit. I'd like to make a few points about Just War, before sticking my oar in (no doubt where it isn't wanted) about some of the more general points raised here - that will be in my next post.
It is possible to wage a "just war", but that will always be subjective. You have to convince yourselves that it is the just fight, because let's face it, you arn't going to convince the enemy are you? Today just war theory is used to justify war to democracies. In the Middle Ages, princes used just war theory to justify their fight to God. I could give you all a history lesson here about the Medieval use of war, but I'll spare you that for today!
So what do we mean by "just war theory"? The theory as it stands today gives seven determiners:
1) Legitimate Authority
2) Just Cause
3) Proportionality
4) Last Resort
5) Prospect of Success
6) Right Intention
7) Just Conduct
These are known as jus ad bellum - justifications for war. Number 7 also covers jus in bello - just conduct, what we would recognise as the rules of war (think Geneva, the Hague etc.).
So for a war to be just, it must first be sanctioned by legitimate authority. In the era of divine right, the LA was God. When the state took on its current sovereign form, LA became the state. It is often argued that post-1945, the UN forms the LA, and most of the time it does, but the State is still the major LA.
2 - Just Cause. This is the big grey area. Your cause must be just. Who decides that? Well, it's all subjective, and certainly not black and white enough to say that self-defence is always right. This is subject to the trends of the day. In the Cold War, fighting communism was "just" as far as the West was concerned. Today, it is spreading democracy. These are incompatible, given that the US toppled legitimately elected governments which were suspected of Communism, but they belong to different ages and were considered at least by some to be "just" in their time.
3 - Proportionality. You can have all the LA you need and the most just cause in the world, but if you act out of proportion, you are in the wrong. This too comes under jus in bello. Let's take an example. If the Falklands War of 1982 was just, which you may or may not believe but let's say it was for this, then that is great. But if the United Kingdom nuked Buenos Aires in the furtherance of her aim (taking the islands back), then it is no longer a just war.
4 - Last resort. War always kills a great deal of people who had no choice. You must have finished the jaw-jaw before you move to the war-war. And finished means actually trying, with the intention of preventing war, not just going through the motions until all your units are in position.
5 - Prospect of Success. A lot of people are going to get killed by beginning a war. If there is no reasonable chance of success, then this will be in vain and the war will have been unjust. Leading your country to certain doom for some sense of honour is not just.
6 - Right Intention. You must enter the war seeking to achieve the just aim. Iraq may have been just, but if Bush or Blair were thinking of oil, or of annexing Iraq, then this invalidates the justness even if their entourages were not thinking these thoughts. Similarly, if Falklands was just but the UK used it as a pretext to invade Argentina, the justness goes out the window.
7 - Just conduct. The laws of war must be adhered to. The USSR may have been justified in kicked Germany out in 1944, but the raping spree embarked upon by its soldiers once they crossed the border negates the justness under this framework.
So that's the modern framework. It is never black and white, even self-defence is not good enough to justify war in itself as a cover all disclaimer. I do believe that wars can be justified, and that this framework is of use to everyone from Christians to Atheists in trying to judge a war "just".
