The Just War Theory is a doctrine, on military ethics, which was mainly a Christian invention. This doctrine was mainly studied by military leaders, theologians, policy makers, etc. The main importance of this doctrine was to ensure a war is morally justified or not. Basically it was like a guide on the principles that made a war considered “just,” or “right.”
St. Augustine was one of the early individuals who studied Just War Theory. His statement on his opinion of the matter was in this quote: War is”justified only by the injustice of an aggressor, and that injustice ought to be a source of grief to any good man, because it is human injustice”.
St. Thomas Aquinas(after St. Augustine), was a highly influential philosopher and theologian, who also had his say on Just War Theory. Aquinas actually further developed the doctrine, and even came up with three conditions that must be met to make a war just. I will be listing these here:
1: you must consider the authority of the sovereign by whose command the war is to be waged.
2: a just cause is required.
3: the individuals involved with the war must have rightful intention, so that they intend the advancement of God, or the avoidance of evil.
Another man who also did some work that further developed Just War Theory, was Francisco Vitoria(1483-1546). He stated that “princes should seek to live at peace,” and also that a just war should be waged within limits. Another thing that he said was that victory should be followed by a spirit of moderation and Christian humility.
So finally, through looking at these figures who developed this doctrine, let us look at some of the main principles of Just War Theory. These principles were: war had to be a last resort, a war had to have a probability of success, noncombatants cannot be targeted, women and children must be left alone, etc. These principles actually made some sense; I mean a war should have a just cause associated with it, war should also have a probability of success because that would be illogical and foolish, and no one who is not involved should be targeted or harmed. So it does make sense why Just War Theory was created in the first place.
In conclusion, the modern “theory,” of war has been influenced by some of the principles of Just War Theory. But none of these “war theories,” or doctrines actually really never fully ensured that a war followed these principles. Many cases throughout history wars ended up being the exact opposite of what Just War Theory’s principles stated to make a war morally “just,” but that is sadly not surprising.