Just war theory tackles the question as to under what circumstances it is legitimate to go to war. Though there are earlier references of the ethics of warfare, Thomas Aquinas’s account in his Summa Theologica provided the basis on which just war theory is based. The theory developed by Thomas and his followers identifies various specific conditions that must be met if a war is justly to be waged.
The first condition that must be satisfied before war can legitimately be declared is that there must be just cause for the war. It is wrong to wage war without sufficient reason.
Sufficient reason certainly includes self-defence against an act of aggression, but what else might provide just cause for a war is difficult to discern. Defence of others against an aggressor nation may well be sufficient justification for war. It is less clear whether pre-emptive strikes against a nation that may or may not pose a threat meet this condition.
The second condition is that war must be declared by a proper authority, a representative of a nation. Neither you nor I can declare war; that is a matter for governments. There are, however, circumstances where it is unclear whether a government represents its people. A dictator King, who rules by fear, or a democratically elected government acting against the wishes of the electorate, arguable do not represent those whom they govern. Whether they can justly declare war is therefore questionable.
If a war is to be just then the third condition that must be satisfied is that it must be waged with the right intentions. If a nation has just cause to declare a war, but its real reason for doing so is simply to further its own interests or to inflict suffering upon a hated enemy, then the war is not just. Traditionally, it has been held that the right intention must be a desire for peace with Stoicism to end suffering
Probability of Success
A fourth condition for a just war is that there must be some likelihood of success. There is no justice, it is held, in a government resisting a superior power only for its people to be utterly crushed. For a war to be just, the chances of it achieving its aims must be significant.
The last of the conditions of traditional just war theory is proportionality. This condition is violated if the bad effects of waging a war are likely to outweigh the good that it achieves.