Aquinas defined the just war. Few conflicts have ever met his criteria
To anyone of humane and pacific instincts, the phrase "a just war" looks like a contradiction. But a moment's thought reveals otherwise. The idea that war, however ugly in itself, is sometimes unqualifiedly just is amply demonstrated by the Second World War, a struggle which provides the focal case of a legitimate use of violence to defend against aggression and put an end to oppression and genocide.
The theory of just war stems from St Thomas Aquinas. In part two of his Summa Theologiae, he examined the proposition "that it is always sinful to wage war", and argued to the contrary that, on three conditions, war can be just. The conditions are: first, that there is a just cause of war, second, that it is begun on proper authority, and third, that it is waged with the right intention, meaning that it aims at "the advancement of good, or the avoidance of evil".
To these conditions, modern theorists have added two others: to be just, a war must have a reasonable chance of success, and the means used to conduct it must be proportional to the ends sought. The first addition is a pragmatic one; one gloss is that a leadership commits an injustice against its own people if it leads them into a war they are sure to lose. The problem with transforming this prudential consideration into a moral one is that it seems, by contrast, rather immoral, not to say spineless, to avoid engaging in an otherwise just war because it threatens to be too costly. When the Polish cavalry galloped towards Hitler's Panzers in defence of their homeland, they were going futilely to war, but their courage gave them a moral victory, and proved an inspiration to others.
The second addition is a controversial one: war leaders otherwise as different in outlook as Churchill and Mao Zedong both emphatically held that once involved in a war, there is no mileage in fighting with one hand tied behind one's back. As Mao succinctly put it, "War is not crochet." Whether this justifies using nuclear weapons or, as in Saddam Hussein's attack on the Kurds of northern Iraq, poison gas, is a very moot point. This involves questions not of what makes it just to go to war, but what counts as acting justly once engaged in war - the difference between the "justum bellum" and the "jus in bello". As the Second World War shows, a war may be a justum bellum and yet lack jus in bello, as the British bombing of Dresden and the American attacks on Hiroshima and Nagasaki arguably illustrate.
There are clear examples of a just cause for going to war. Defence against aggression is one, as is rescuing people being subjected to aggression. Is it equally just to engage in pre-emptive military action against a potential aggressor? History teaches that appeasement and inaction are dangerous tactics; but how can one be sure that an unfulfilled threat is genuinely dangerous, and can one be sure that alleging a threat is not a mask for one's own aggression? On the other hand, a people has a right to defend itself, and the best form of defence is to prevent attack in the first place - by diplomacy if possible, but by force if necessary. A leadership fails its people if it does not prevent them from being harmed by aggressors.
What might count as just war aims? Aquinas said: promoting the good or avoiding evil. By this is meant that a war waged for such reasons as self-interest, "Lebensraum", other people's oil fields, or pure aggrandisement, is not justified. "Avoiding evil" can be invoked by those seeking to defend themselves preventatively, and - if they are right about the threat posed by a delinquent regime amassing dangerous weapons - with justification. The positive aim, that of promoting good, which must at least involve bringing about peace, stability, democracy, prosperity, and a situation in which both victor and defeated can cease to be enemies, is equally easy to identify. Too many wars, whether just in their inception or not, fail to achieve this outcome, because the harder struggle of "winning the peace" is too often fudged or dodged.
Although the Aquinas conditions are clear and compelling, they can never count as sufficient conditions for going to war. Other considerations apply. Have all diplomatic means failed? Is there consensus over the "just cause" and "right intentions" requirements among all parties implicated on the side of the intending belligerent? In practice, diplomacy and various forms of pressure, including sanctions, are the standard means for adjusting international disagreements, and in most cases actual military conflict is a last resort, whether just or not. Certainly, it seems hard to describe a war as just if it is not a last resort.
The truly just war would be the war of ideas which roots out the greed, stupidity, superstition and ignorance which ultimately lie at the roots of all human conflicts. Only then will it be possible to beat swords into ploughshares and shell casings into scales of justice. Until that time, people will fight - sometimes, with justice, for freedom and life.