There is a long-standing tradition in Western culture of differentiating between just and unjust wars. Although people who are opposed to war in principle will surely disagree that any such distinction can possibly be made, the basic ideas involved seem to present a plausible argument that there are times when war is, at the very least, less just and as a result should receive less support from the public and from national leaders.
The basic starting point of Just War Theory is that while war may be awful, it is nevertheless sometimes a necessary aspect of politics. War does not exist outside of moral deliberation - neither the argument that moral categories do not apply nor the claim it is inherently a moral evil is convincing. Therefore, it must be possible to subject wars to moral standards according to which some wars will be found more just and others less just.
Just War theories were developed over the span of many centuries by a variety of Catholic theologians, including Augustine, Thomas Aquinas, and Grotius. Even today most explicit references to a Just War theory are likely to come from Catholic sources, but implicit references to its arguments may come from anywhere because of the way in which it has become incorporated into Western political principles.
How do Just War theories expect to justify the pursuit of some wars? How can we ever conclude that some particular war may be more moral than another? Although there are some differences in the principles used, we can point to five basic ideas which are typical. Anyone advocating a war has the burden of demonstrating that these principles are met and that the presumption against violence can be overcome. Although all have obvious relevancy and value, none are easy to employ because of inherent ambiguities or contradictions.
Just War theories definitely have some difficulties. They rely upon ambiguous and problematic criteria which, when questioned, prevent anyone from readily applying them and concluding that a war definitely is or is not just. This does not, however, mean that the criteria are useless. Instead, it demonstrates that ethical questions are never clear-cut and that there will always be grey areas where well intentioned people will not necessarily agree.
The criteria are helpful in that they provide a sense of where wars can go wrong, assuming that they arent inherently wrong to begin with. Although they may not define absolute boundaries, at the very least they do describe what nations must strive towards or what they must move away from in order for their actions to be judged reasonable and justified.