What is Just War Theory?

Legal Definition
Just war theory (Latin: jus bellum iustum) is a doctrine, also referred to as a tradition, of military ethics studied by theologians, ethicists, policy makers, and military leaders. The purpose of the doctrine is to ensure war is morally justifiable through a series of criteria, all of which must be met for a war to be considered just. The criteria are split into two groups: "right to go to war" (jus ad bellum) and "right conduct in war" (jus in bello). The first concerns the morality of going to war, and the second the moral conduct within war. Recently there have been calls for the inclusion of a third category of Just War theory—jus post bellum—dealing with the morality of post-war settlement and reconstruction.

Just War theory postulates that war, while terrible, is not always the worst option. Important responsibilities, undesirable outcomes, or preventable atrocities may justify war.

Opponents of Just War theory claim that there has never been a political philosopher who argued that war poses any real benefit to any participating party. In a large number of cases, philosophers state that individuals need not be of guilty conscience if required to fight. A few nobilify the virtues of the soldier while declaring spite for war itself. A few, such as Rousseau, argue for insurrection against oppressive rule.

The historical aspect, or the "just war tradition," deals with the historical body of rules or agreements that have applied in various wars across the ages. The just war tradition also considers the writings of various philosophers and lawyers through history, and examines both their philosophical visions of war's ethical limits and whether their thoughts have contributed to the body of conventions that have evolved to guide war and warfare.
-- Wikipedia