Explanation:
The just war theory is a largely Christian philosophy that attempts to reconcile three things:
taking human life is seriously wrong
states have a duty to defend their citizens, and defend justice
protecting innocent human life and defending important moral values sometimes requires willingness to use force and violence
The theory specifies conditions for judging if it is just to go to war, and conditions for how the war should be fought.
Although it was extensively developed by Christian theologians, it can be used by people of every faith and none.
Purpose
The aim of Just War Theory is to provide a guide to the right way for states to act in potential conflict situations. It only applies to states, and not to individuals (although an individual can use the theory to help them decide whether it is morally right to take part in a particular war).
Just War Theory provides a useful framework for individuals and political groups to use for their discussions of possible wars.
The theory is not intended to justify wars but to prevent them, by showing that going to war except in certain limited circumstances is wrong, and thus motivate states to find other ways of resolving conflicts.
'Just', or merely 'permissible'?
The doctrine of the Just War can deceive a person into thinking that because a war is just, it's actually a good thing.
But behind contemporary war theory lies the idea that war is always bad. A just war is permissible because it's a lesser evil, but it's still an evil.
Origins
The principles of a Just War originated with classical Greek and Roman philosophers like Plato and Cicero and were added to by Christian theologians like Augustine and Thomas Aquinas.
Elements
There are two parts to Just War theory, both with Latin names:
Jus ad bellum: the conditions under which the use of military force is justified.
Jus in bello: how to conduct a war in an ethical manner.
A war is only a Just War if it is both justified, and carried out in the right way. Some wars fought for noble causes have been rendered unjust because of the way in which they were fought.
