Augustine of Hippo (354 - 430) first developed the requirements for a just war, and Thomas Aquinas (1225 - 1274) later refined them. Personally, I hold beliefs about war more in line with pacifism than with just war theory. Nevertheless, for those without pacifist leanings, these should represent the bare minimum considerations for war-making.
1. Last Resort
A just war can only be waged after all peaceful options are considered. The use of force can only be used as a last resort.
2. Legitimate Authority
A just war is waged by a legitimate authority. A war cannot be waged by individuals or groups that do not constitute the legitimate government.
3. Just Cause
A just war needs to be in response to a wrong suffered. Self-defense against an attack always constitutes a just war; however, the war needs to be fought with the objective to correct the inflicted wound.
4. Probability of Success
In order for a war to be just, there must be a rational possibility of success. A nation cannot enter into a war with a hopeless cause.
5. Right Intention
The primary objective of a just war is to re-establish peace. In particular, the peace after the war should exceed the peace that would have succeeded without the use of force. The aim of the use of force must be justice.
6. Proportionality
The violence in a just war must be proportional to the casualties suffered. The nations involved in the war must avoid disproportionate military action and only use the amount of force absolutely necessary.
7. Civilian Casualties
The use of force must distinguish between the military and civilians. Innocent citizens must never be the target of war; soldiers should always avoid killing civilians. The deaths of civilians are only justified when they are unavoidable victims of a military attack on a strategic target.