Thomas Aquinas was a Christian Theologian and created the theory of Just War which has set conditions for when Christians should be okay with allowing war.
The conditions are split into Jus Ad Bellum-Right to go to war, and Jus in Bello-Conduct in war.
Jus Ad Bellum: Right to go to war
*There must be a just cause for going to war:
Defending life/protecting human rights or securing justice are good examples
"It is not good to be partial to the wicked and so deprive the innocent of Justice"-Proverb 18:5
*War must be declared by a competent legal authority
This is usually the local government
"Let everyone be subject to the governing authorities... The authorities that exist are established by God"-Romans 12:1-6
*There must be a comparison of justice on both sides
This is usually difficult to achieve as both sides will maintain the idea of having a just cause.
*There must be right intention
The right intention must be as just as the cause and not be out of hatred/revenge or anger
"All people's actions seem pure to them, but motives are weighted by the Lord"-Proverb 16:2
*It must be a last resort
After all other attempts of peace have been made i.e. negotiations and military sanctions.
"Do not repay anyone evil for evil. Be careful to do what is right in the eyes of everyone"-Romans 13:17-18
*There must be proportionality
Between the injustice occurring and the suffering that war would cause and the cause but not b upheld by unjust means.
*There must also be a likelihood of success.
So the outcome results in a better state of affairs.
"Suppose the King is about to go to war against another King. Won't he sit down and consider whether he is able with ten thousand men to oppose the one coming against him with twenty thousand?" Luke 14:31
Just In Bello: Conduct in War
*Proportionality
The amount of weapons must be proportional to the threat. Only minimum force should also be used.
*Discriminate
Civilians should be protected and not attacked. The deliberate intention of killing civilians is prohibited. Th use of nuclear weapons renders this criteria impossible.
"Have nothing to do with a false charge and do not put an innocent or honest person to death, for I will not acquit the guilty"-Exodus 23:7
