What is commonly known as "Just War" has changed a bit over time, but one of the key theologians/philosophers in developing this doctrine was Thomas Aquinas (b.1225). He came up with three different criteria to go to war (Remember, there is a whole different criteria for a just way to act during war):
1) The war can only be waged by a soveriegn nation or governing body, not random individuals. So if there is a system in place, the declaration of war must go through this system. (This one seems pretty clear)
2) There must be a just cause- (This is obviously what people debate all the time). Often times this comes into play when the nation declaring war is threatened, protecting themselves, preventing serious evils or dangers. (This would be a main topic of your argumentation essay. What counts as a"just cause?")
3) Aquinas then said that lastly, the nation declaring war must have just "intentions," meaning their intentions must promote good and not evil when declaring war. The intentions have to be right. (This is also greatly contentious because a lot of nations think they may have the right intent, but do they? What would be the true intention for going to war? Many people would argue that nations only declare war because of some selfish intentions.)
Over the next few hundred years, other parts have been added by theologians, but I think these three will be enough to focus on for your essay.
A couple interesting sources:
https://www.youtube.com/watch?v=b8lJqBVOQ10
http://catholiceducation.org/articles/religion/re0182.html
Actual Text from Aquinas's Summa Theologica:
I answer that, In order for a war to be just, three things are necessary. First, the authority of the sovereign by whose command the war is to be waged. For it is not the business of a private individual to declare war, because he can seek for redress of his rights from the tribunal of his superior. Moreover it is not the business of a private individual to summon together the people, which has to be done in wartime. And as the care of the common weal is committed to those who are in authority, it is their business to watch over the common weal of the city, kingdom or province subject to them. And just as it is lawful for them to have recourse to the sword in defending that common weal against internal disturbances, when they punish evil-doers, according to the words of the Apostle (Romans 13:4): "He beareth not the sword in vain: for he is God's minister, an avenger to execute wrath upon him that doth evil"; so too, it is their business to have recourse to the sword of war in defending the common weal against external enemies. Hence it is said to those who are in authority (Psalm 81:4): "Rescue the poor: and deliver the needy out of the hand of the sinner"; and for this reason Augustine says (Contra Faust. xxii, 75): "The natural order conducive to peace among mortals demands that the power to declare and counsel war should be in the hands of those who hold the supreme authority."
Secondly, a just cause is required, namely that those who are attacked, should be attacked because they deserve it on account of some fault. Wherefore Augustine says (QQ. in Hept., qu. x, super Jos.): "A just war is wont to be described as one that avenges wrongs, when a nation or state has to be punished, for refusing to make amends for the wrongs inflicted by its subjects, or to restore what it has seized unjustly."
Thirdly, it is necessary that the belligerents should have a rightful intention, so that they intend the advancement of good, or the avoidance of evil. Hence Augustine says (De Verb. Dom. [The words quoted are to be found not in St. Augustine's works, but Can. Apud. Caus. xxiii, qu. 1): "True religion looks upon as peaceful those wars that are waged not for motives of aggrandizement, or cruelty, but with the object of securing peace, of punishing evil-doers, and of uplifting the good." For it may happen that the war is declared by the legitimate authority, and for a just cause, and yet be rendered unlawful through a wicked intention. Hence Augustine says (Contra Faust. xxii, 74): "The passion for inflicting harm, the cruel thirst for vengeance, an unpacific and relentless spirit, the fever of revolt, the lust of power, and such like things, all these are rightly condemned in war."
