There is no reason to assume, for example, that simply because we believe that war can be a purposeful and reasoned activity that we will be inclined to engage in it too easily. In fact, defenders of Christian Just War doctrine typically argue that we ought to be reluctant to fight wars that lack sufficient moral and rational justification. Defenders of the Just War tradition regret that they live in a world where they have to kill human beings in order to restrain evil; that is to say, they regret the Fall. But they find it to be even more regretful for Christians to stand idly by while people are being abused and killed unjustly.
Of course people regret having to go to war, having to kill people, but that doesn't mean they wouldn't do it again given the same situation. If someone breaks into your home and tries to kill your family, and you get the opportunity to kill them, you are not going to regret it or feel you've done something wrong. You will probably regret that the situation arose, that you were put into that situation, but given that situation you did the right thing. There is a huge difference between regretting a situation existed, and regrettting your actions given that situation.
Of course, war is always contrary to peace, but this is sometimes desirable, since peace is not always a just order that deserves to be preserved. Nazi Germany, for example, provided peace and order for most of those in conquered countries who were willing to accept Nazi rule. But no one wishes to argue that the peace provided by Nazis is the sort of peace we ought to preserve. War, for Aquinas, can be a means to a just peace as well as a means to destroy an unjust peace (such as one established by Nazis). We keep a just peace and fight just wars because these are acts of charity.
This is one thing that really bothers me about a lot of 'peace at all costs' type people. It seems that so many are more concerned about peace and the perception of safety that they are willing to sell their rights, and the rights of others, down the river. There was a time when justice and freedom were far more important. They might as well be saying we shouldn't have fought the American Revolution, the Civil War, or even stopped the Nazis in WWII. In none of these cases were the majority of the people in imminent physical danger. However they were the most just of wars, the Civil War being long overdue. The fact of war may not be 'good', but it certainly is often 'good' compared to what not going means.
The issue of how we fight is another matter. Aquinas himself offers little guidance regarding the proper rules for fighting (called jus in bello). Nevertheless, we can extrapolate a handful of guidelines from his writings. For one thing, we can presume that we should fight with the right intention, that is, we must intend to punish not just anyone, but only evildoers. Likewise, we should do our best to see that our use of force does not detract from our duty to uphold the good. Of course, the ability to target only those who deserve to be punished, no less than the capacity to formulate plans of action that will issue in more good than evil, must be cultivated. Thus, for Aquinas, right conduct in war is dependent upon the virtues of soldiers and the commanders who lead them.
The most noteworthy aspect of the moral approach to warfare in Aquinas and Calvin is that it teaches—contrary to today’s prevailing views—that a failure to engage in a just war is a failure of virtue, a failure to act well. An odd corollary of this conclusion is that it is a greater evil for Christians to fail to wage a just war than it is for unbelievers. When an unbeliever fails to go to war, the cause may be a lack of courage, prudence, or justice. He may be a coward or simply indifferent to evil. These are failures of natural moral virtue. When Christians (at least in the tradition of Aquinas and Calvin) fail to engage in just war, it may involve all of these natural failures as well, but it will also, and more significantly, involve a failure of charity. The Christian who fails to use force to aid his neighbor when prudence dictates that force is the best way to render that aid is an uncharitable Christian. Hence, Christians who willingly and knowingly refuse to engage in a just war do a vicious thing: they fail to show love toward their neighbor as well as toward God.
In an example that people here could probably relate to, Gabrielle on Xena always bothered me. At times I outright couldn't stand her. I thought it hypocritical that she wouldn't kill, but was more than willing to let Xena do it for her. And it also bothered me how, on a number of occasions, she stood by while innocents and her friends were being attacked, and was totally unwilling to help them. That is immoral. And unfortunately it was a very realistic portrayal of many people.
And of course, there are the individuals and nations in Europe during WWII who were more than willing to sacrifice others (neighbors and friends) in order to save themselves from violence (they thought).
I wish people would stop whining "why can't we just get along" . That day will never come. There will always be 'broken' people, and some of them will always rise to power and find others to follow them, or force others to follow them. The goal is to reduce how often that happens by making a better world where people understand justice and freedom. The goal was never peace, it was freedom and justice, with the hope that those standards would create a better and more peaceful world, eventually. But there will always be those who don't want that, who want power, and who enjoy violence; the rest of us will always have to deal with them or risk everything by ignoring them to long.
And for people to say that peace is best, when people are being tortured and killed on a daily basis in that peace, are clueless at best, selfish at worst.
I find it amazing that some people claim to be worried about the poor people of Iraq that will be killed in war, but don't seem to care about the poor people of Iraq that are already dying and suffering every day. Makes no sense, especially when the war will end that suffering.