One can argue that by its ordinary Magisterium the Church has infallibly defined that wars can be just, but it has never tried to infallibly define the precise conditions that must be met for a particular war to count as just. The conditions enumerated in the Catechism of the Catholic Church (CCC 2309) are certianly true in their broad outline, but there remains potential for doctrinal development in this area.
On thing that may force that doctrinal development to occur would be the advent of widespread use of non-lethal weapons.
Church thought on just warfare has been affected by developments in weapons technology before. When the crossbow was developed in the Middle Ages, Pope Urban II forbade its use against Christians. So did the Second Lateran Council, which stated: “We prohibit under anathema that murderous art of crossbowmen and archers, which is hateful to God, to be employed against Christians and Catholics from now on” (Canon 29).
Current attitudes toward warfare are heavily shaped by the experience of World War II, which ravaged Europe the current generation of Vatican officials were young, and the Cold War, which threatened to turn nuclear when they were in middle age. The assumption presently made in Vatican circles is that increasing technology ineluctably makes warfare more and more deadly, making it harder and harder to justify. This perception is displayed at a number of places in the Catechism, including in the conditions of a just war, which state in part:
The use of arms must not produce evils and disorders graver than the evil to be eliminated. The power of modem means of destruction weighs very heavily in evaluating this condition.
Certainly the possession of modern means of destruction weighs heavily on anyone planning to use them, but the concept the Church is expressing needs to be further refined. It is not a given that better weapons automatically lead to deadlier wars. The recent wars in Afghanistan and Iraq, for example, were both far less deadly for both sides and for the civilian population than would have been the case had they been fought in the 1940s, when carpet bombing was commonly imployed to get at munitions factories within urban areas. Modern “surgical” warfare may still be messy, but it is far less deadly.
Studies have been done on the casualties caused by warfare at different technological levels and the results are quite clear: The more primitive the weapons are, the more people get killed by war. It is in the most primitive societies that the highest percentage of the population dies by warfare, and it is in the most technologically advanced societies that the fewest people die. (NOTE: I know people will be curious about these studies, so I’ll post more information on them soon.)
One of the reasons for this, undoubtedly, is that the more lethal you and your opponents’ weapons are, the more carefully you are going to think about whether you really want to go to war, the more motivated you will be to find options other than warfare to settle your differences. Another reason is that it is possible to apply force more precisely and have fewer casualties as collateral damage.
What remains to be seen is what effect the development and widespread deployability of non-lethal weapons will have. Paradoxically, it might have the effect of making wars even less deadly–but more common.
All of this is likely to give Catholic moral theologians fits, and the debate will rage for several generations. Doctrinal development seldom happens quickly, and do not expect it to happen on this question for years to come. As long as the destruction of World War II and the terror of the Cold War remain living memories for Vatican officials, a re-examination of this question will be out of the question.