Just war requires:
1. Just Cause: The OT allows for self-defense on the personal level (Exo. 22:2), and on the local social level with the right to capital punishment for prescribed crimes (Exo. 21:12), and this is extended to the level of national self-defense against assailing enemies: Exo. 17:8-9; cp. 1 Sam. 30:3, 18-19; Deut. 28:7. The NT commends just war by placing in the “Hall of Faith” those who “became mighty in war, put foreign armies to flight” (Heb. 11:34).
2. Last Resort: All reasonable non-violent options must be exhausted before the use of force can be justified: Deut. 20:10-12. Israel had God-defined borders (Gen. 15:18; Ex. 23:31; etc.) and could never legitimately possess imperialistic pretensions. This passage is not dealing with the special, limited Holy War which secured the Promised Land: “Thus you shall do to all the cities that are very far from you, which are not of the cities of these nations nearby” (Deut. 20:15).
3. Legitimate Authority: According to Rom. 12:19, vengeance belongs to God who will repay the evildoer. Just three verses later Paul says that God has given the right to avenge wrongdoing to the civil magistrate who is the “minister of God” in this respect ( Rom. 13:1-4). “Governors [are] sent by him for the punishment of evildoers and the praise of those who do right” (1 Pet. 2:14), which includes punishment of whole nations that threaten evil against another nation.
4. Successful Prospect: A war can only be just if it is fought with a reasonable chance of success. Human life is precious, in that man is created in the image of God (Gen. 1:26-27). God has a special concern for man, His highest creature: Ps. 8:4. God ordains the protection of human life: Gen. 9:6. This, of course, prohibits suicide, even at the national level. In a parable, Jesus touches on this principle of war: Luke 14:31. David expresses a concern for the prospect of utter defeat: 1 Sam. 30:8. The righteous seek safety, not destruction: 2 Sam. 22:3-4. Consequently, wide-scale deaths and injury incurred in a hopeless cause are not morally justifiable, being a form of national suicide.
5. Peaceful Objectives: The ultimate goal of a just war is to re-establish peace and safety. The historical goal of the Kingdom of God clearly teaches this primary historical objective: Isa. 2:4.
6. Proportionate Means: The violence meted out in war must be proportional to the injury suffered. For instance, the laws governing capital punishment constrain the state by not allowing the magistrate to capitally punish a thief (Ex. 22:7) or to put to death the murderer’s family (Deut. 24:16). Likewise, the aim of war must be constrained by principles of proportionality, according to the lex talionis principle of “an eye for an eye” (Exo. 21:24; Lev. 24:20f). Total war is prohibited, either against man or against his land: Deut. 20:19-20.
7. Civilian Immunity: Civilian deaths are tolerable only as accidental, unavoidable collateral damage resulting from an attack on a legitimate military target: Jer. 51:3; Deut. 20:13-14. Again, this embodies the principle of man being the image of God and under His protection.
- Kenneth L. Gentry, Jr., "Just War" Theory, Faith for All of Life (Dec. 2004) at http://www.chalcedon.edu/articles/article.php?ArticleID=2369.