The just war theory is an attempt to distill moral principles into guidelines relevant to warfare. The goal of the just war theory is to identify when it is morally acceptable to support a particular conflict. From a Christian standpoint, this means applying biblical ideas to create those practical standards. The theory of a “just war” is not exclusively Christian. Versions of this idea were common in Greek and Roman civilization. However, spirituality strongly influences how just war theory is applied. Pagan definitions of a just war typically considered expansion and revenge as acceptable motivations. Some faiths, such as Jainism, adhere to absolute pacifism. Others, such as Islam, were founded by men who frequently waged wars of aggression. Biblical Christianity presents an approach to war significantly more nuanced than that of most other religions.
Briefly stated, just war theory says that armed conflict is only moral as a last resort, waged by a legitimate government, for moral reasons, and using moral means. As with most real-world issues, whether or not a particular conflict meets just war criteria is always subject to debate. It’s also critically important to distinguish between the concept of a just war and the idea of a holy war. Just war theory does not support the concept of waging holy wars, and neither does the Bible.
Christian application of just war theory stems from several scriptural principles: human beings have intrinsic value (Genesis 1:27) but are also inherently sinful (Romans 3:10). God instituted human government specifically to maintain order and justice (Romans 13:1–5; 1 Peter 2:14). Mankind in general, and Christians in particular, are morally obligated to pursue a more just world (Proverbs 21:3; Micah 6:8; Matthew 5:13–16). This obligation does not, however, imply any use of violence to “advance” the faith (John 18:36). Further, God’s prohibition on killing applies to murder (Exodus 20:13), not to capital punishment (Genesis 9:6) or justified warfare (Psalm 18:34) or legitimate self-defense (Luke 22:36). At the same time, cruelty, revenge, and hatred are condemned by the Bible (Romans 12:19; Proverbs 20:22; Galatians 5:19–24).
The most commonly understood version of just war theory is grounded in these biblical ideals, simplified into five major points. Per just war theory, any conflict not meeting all of these conditions is “unjust” and morally unacceptable. It’s important to re-emphasize that this framework cannot remove all possible controversy. There will always be differences of opinion about whether a particular war—or any war—fits these points.
The five main requirements of a “just war” are as follows:
1. A just war is declared by a legitimate government. According to just war theory, independent people or groups cannot act as vigilantes, taking upon themselves the right to wage aggressive warfare. This also excludes government actions that go beyond established rules; for instance, if a national leader were to ignore that country’s laws in ordering an attack or if a military leader staged a coup and immediately attacked another nation. Also, war—including the intention to attack—must be formally and officially announced before a nation engages in hostilities. This provides additional opportunities to resolve a dispute, warns civilians who might be affected, and further forces the government to validate violence beforehand.
2. A just war is an act of last resort. Prior to engaging in violence, a nation must make every effort to attain its intended goals by other means. This might include diplomacy, economic or legal actions, and so forth. This is a crucial tenet of just war theory: war results from the failure of all other options. It is not one option among many. As an extension of this idea, the government should seek to end the conflict as quickly as is reasonably possible.
3. A just war is fought for a just cause. The intended outcome of the war itself must be morally upright. Goals such as freeing people from death and persecution or stopping another nation’s conquest might meet this definition. A desire to gain more land or to punish another nation for some perceived insult would be an example of an unjust goal.
4. A just war seeks prudent goals. Warfare is less justifiable when it has little to no chance of succeeding. This requirement is meant to balance the concept of a “just cause.” Grandiose ideas can’t be claimed as valid reasons for bloodshed. A tiny nation is foolish to launch an invasion of a military superpower—the effort is virtually guaranteed to fail, making the resulting mayhem all but pointless. Likewise, a war cannot be justified unless its goals, even noble ones, are proportionate to its toll of death and destruction. For example, the goal of improving another nation’s educational system is not a morally valid reason to engage in open warfare.
5. A just war uses moral means. According to just war theory, noble ends or goals do not justify any and all actions to achieve victory. In short, the ends do not justify the means. This means a just war is restrained to proportionate levels of violence and does not engage in excessive or cruel use of force. This principle also requires making an effort to avoid civilian casualties, undue destruction, or actions that would unreasonably affect those uninvolved in the conflict. “Moral means” extends to details such as the treatment of captured soldiers and civilians and efforts to reconcile after the conflict is over.
Virtually every violent conflict in human history was labeled “justified” by the side that started it. Obviously, this claim is often false. And yet, a valid moral principle is not invalidated because it is not followed. On the contrary, examples of wars fought unjustly show how just war theory can reduce violence rather than encourage it.
Just war theory acknowledges that war is not a good thing—the five requirements are specifically meant to circumvent any violence and mayhem unless it is unavoidable. In sum, just war theory treats war as something unfortunate and unpleasant, even in the midst of the conflict. Humane and merciful treatment of one’s enemies is, in particular, an example of how Christian ethics have influenced just war theory.
A common misconception concerning just war theory is that Christianity uses Israel’s battles in the Old Testament as excuses for modern attitudes toward war. Certainly, broad principles can be drawn from God’s use of warfare in the Old Testament. Some are, indeed, reflected in the modern concept of just war (Deuteronomy 20:10, 19). However, Scripture is clear that Israel’s war in Canaan was not a model for future conquest (Deuteronomy 9:6); neither Judaism nor Christianity has attempted, at large, to apply the conquest of Canaan in that way. Likewise, from a Christian standpoint, there is never justification for war or violence in an attempt to promote, expand, or spread the gospel (John 18:36; 2 Corinthians 10:4). Such efforts are inherently contradictory to the faith.
Views regarding just war theory are closely tied to a person’s view of self-defense. When may a Christian use violence to defend himself? Just war theory teaches that, beyond a personal, defensive level, the involvement of government is an absolute necessity for justifiable fighting.
Just war theory is by no means the “official” position of Christianity on this subject. It is, however, the most common approach found among Christian denominations. Within biblical Christianity, just war theory is more likely to be rejected by those who favor a more pacifist interpretation of Scripture. There are few, if any, Christian groups who overtly favor aggressive or predatory warfare. The reason for the broad acceptance of just war theory within Christianity is reflected in the fundamental assumption of the theory itself: that violence may at times be necessary but only as the last resort.