When a war breaks out, we want to know that the action was justified. So, when is it justified to use military force? In other words, when is war morally permissible?  “Just War Theory” was primarily established by Augustine in the 5^th century, and further developed by Thomas Aquinas in the 13^th century. It is the Christian understanding of when war is morally justified, and how a morally justified war should be undertaken.

On a recent episode of The Briefing, Albert Mohler shared seven criteria that have to be met for a war to be just and justly fought. He stated:

1. Just Cause. There must be just cause. You can’t just declare war certainly aggressively on another nation because you want its territory. You have to have just cause. There has to be some defensive, basic defensive move that is made necessary in terms of the use of violence. Repelling an invader is legitimate. Invading another country because you want their goods is not legitimate.
2. Legitimate Authority. Military action must be undertaken by legitimate authority. This means in the case of the United States, legitimate authority according to the legal precepts of the United States, the Constitution of the United States. Of course, that gets a little complicated.
3. Right Intention. There must be right intention. This seems somewhat subjective. The intention in undertaking military action, the use of lethal force, it has to be driven by something that is righteous. Now remember, Just War Theory is about when war is just when it is justified, when it meets the demands of justice. When you’re looking at right intention, this means right as in righteous. It must be a righteous intention such as self-defense or protecting the innocent.
4. Last Resort. It must be the last resort. Other things have to be tried. Negotiation has to be tried. Diplomacy has to be tried. Some nonviolent, let’s say, settlement of these differences ought to be tried. Now, the fact is, however, in a fallen world, sometimes not only is such a thing impossible, it’s made impossible by something like a surprise attack. This is what happened in the case, of course, of Japan against the United States in 1941. There really, at that point, was no question that military action was a last resort.
5. Reasonable Hope for Success. There must be a reasonable hope for success. So that is to say, you don’t just throw armies into a certain death. You do not waste human life if there’s not a reasonable hope of success.
6. Proportionality. There must be proportionality when it comes to military operations. That’s to say if one side shoots arrows, the other side doesn’t have the right to drop an atomic bomb. That’s an extreme example, but you get the point. There has to be proportionality.
7. Establishment of a Just Peace. There must be the establishment of a just peace on the other side. The war must be fought by just means with the goal of achieving a just peace.

In his teaching series “The Just War”, R.C. Sproul taught that while war is a result of evil, it is sometimes necessary to stop an aggressor and protect the innocent. He taught that Scripture permits war only when fought for right reasons by legitimate authority, acting as a “necessary force” to restrain evil.

In an article titled “When to War”, Ligonier Ministries stated that Just War Theory is based on two major elements:

1. All war is the result of evil. Before the fall, there was no strife between human beings. Yet the hostility evident immediately after Adam and Eve sinned (Gen. 3:12–13) has only multiplied as the human population has increased. Today, coveting power or resources produces most armed conflicts. Even the divinely instituted conflict against Canaan resulted from sin, for had the Canaanites not sinned, God would not have needed to judge their wickedness.
2. Fighting in a war is not necessarily evil. Scripture gives us the right of self-defense (Ex. 22:2–3), and war can be self-defense on a grand scale. As Romans 13: 1-7 teaches, God gives the government, not private citizens, the authority to administer punitive justice. Wars can be declared only by governments, and thus, believers can join the army and defend their country against illegal assaults. This does not forbid preemptive strikes in the case of a clear and present danger, but preemption must be used very cautiously.

Other caveats do apply. A nation should go to war only after all other attempts to resolve the conflict peacefully have failed (Deut. 20:10–15). There also is to be proportionality; the just party must endeavor not to harm non-combatants and must only respond to an attack in kind. For example, a nation should not use atomic weaponry against a poorly armed militia (v. 14).

The above is intended only as an introduction to Just War Theory. Until Jesus returns, there will always be wars. Christians want to know that those wars are just.