The earliest Christians were pacifists. They took Christ's words very seriously - to love one's enemies, to turn the other cheek, to put down the sword. Living in an empire that was persecuting them perhaps made it easier for them to refuse to fight for the state. That changed when Constantine became a Christian (312 A.D.), and thus, so did the Roman Empire.
Therefore it fell upon Augustine and later Aquinas to work out the ethics involved in a "just war," one that Christians could support and fight in in good conscience. Here's what they came up with:
Principles of Just-War Theory
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.
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 militia 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.
Prayer: God of Peace, may our wars be few and when they cannot be avoided may they be just. Guide us in the paths of peace and justice. Teach the nations of the world a better way to work out our differences. In Your Holy Name. Amen.
Extra Credit: Food for thought:
Does modern warfare (smart bombs, drones, cyber attacks, etc.) help or hurt compliance with the principles of the "just war" theory?