Wilson at "The Elfin Ethicist" has an interesting post on hating war.
I tend to a (relatively medieval) just war theory myself; but this gives less room for supporting war than one might think. The reason is this.
As Wilson notes, when we talk about 'war' we may mean either an action or an event. This is actually very important, because there are really two things called 'just war theory' that are extremely different, and they divide more or less along this line. One 'just war theory' means by 'war' the event of going to, and being in war; the other means by it the personal action of warring. There is all the difference between them.
When Aquinas writes on just war, he is talking about one thing, and one thing only: Given that warring involves fighting one's enemy, and given that Christians are supposed to love their enemies, can it ever be just for a Christian prince to war? His answer is that it can be if (1) the prince has been put in charge of protecting the people, and thus has legitimate authority; (2) the prince is going to war because his enemies actually deserve it; (3) the prince is properly disposed, i.e., virtuous in his means, to that end. The idea behind all three is the common good; a prince put in charge of protecting the common good of all, must deal with threats to it, and that includes external as well as internal enemies. Aquinas is concerned entirely with the prince as the person who actually wars; soldiers are only barely brought in insofar as they participate in the prince's warring through obedience; and civilians not at all.
Eventually people began to think of war (the situation) when they talked about 'just war'; and this, I think, goes with another modern innovation, the notion of total war. That is, we have made it virtually impossible for ourselves not to think of nations as going to war. This would have been utterly unthinkable for Aquinas, except perhaps as a figure of speech. He thinks of war as a heavy external policing action; so the whole responsibility for it, and the whole issue of the justice for it, lies entirely with the person in charge. We don't hold the whole nation responsible for a major SWAT operation, even if it were a very extended and elaborate one. It would never have occurred to him that anyone could think that, when the king of France goes to war against the king of England, the peasants of France are "in a state of war" with the peasants of England; warring is just not the sort of thing peasants do. But we do think of it in these terms. And this is, perhaps, a problem from the (older) just war perspective. While it's always been the case that civilians could get caught in the crossfire of two leaders, it is our innovation to think of one civilian population warring against civilian another as an integral part of war. But it is immoral, unjust, uncharitable, to war against someone who doesn't deserve it; and it isn't clear that there is any real sense in which we can say an entire nation deserves to be warred against. This suggests, that, while there are still some things (a very few) that we might call 'war' that would be allowed in themselves, the older just war theory, which I think is the better-founded one, would hold us to be engaging in an act of injustice even in those cases because of the way we think about war - we (and this includes civilians as well as leaders and soldiers) are treating people as our enemies who don't deserve it, and that's unjust. In other words, virtually any war today (and this will include much of what has to do with war, e.g., international law and peacekeeping) is accompanied with an immense amount of injustice because the way we think about war encourages us to be unjust toward others.
I'm not sure this was stated very clearly (I'm rushing to get the post out before class), but I think this highlights a problem for us. I don't think this is unanswerable; but I do think answering it would require a complete rethinking of the way we actually approach the whole notion of war, and indeed, all military action.