Here's my version of Augustine's Just War Theory (probably derived from a source that I can no longer cite). It is a stance which historic Christianity has held for centuries, although it is not considered binding, and every Christian must determine his or her own stance on the issue of war.
1. A just war can only be waged as a last resort. All non-violent options must be exhausted before the use of force can be justified.
2. A war is just only if it is waged by a legitimate authority. Even just causes cannot be served by actions taken by individuals or groups who do not constitute an authority sanctioned by whatever the society and outsiders to the society deem legitimate.
3. A just war can only be fought to redress a wrong suffered. For example, self-defense against an armed attack is always considered to be a just cause (although the justice of the cause is not sufficient--see point #4). Further, a just war can only be fought with "right" intentions: the only permissible objective of a just war is to redress the injury. A country may not justly start a war to grab another country’s assets such as precious minerals or other natural resources.
4. A war can only be just if it is fought with a reasonable chance of success. Deaths and injury incurred in a hopeless cause are not morally justifiable.
5. The ultimate goal of a just war is to re-establish peace. More specifically, the peace established after the war must be preferable to the peace that would have prevailed if the war had not been fought.
6. The violence used in the war must be proportional to the injury suffered. States are prohibited from using force not necessary to attain the limited objective of addressing the injury suffered.
7. The weapons used in war must discriminate between combatants and non-combatants. Civilians are never permissible targets of war, and every effort must be taken to avoid killing civilians. The deaths of civilians are justified only if they are unavoidable victims of a deliberate attack on a military target.