While the early church was originally pacifist, from the time of Augustine it started to develop Just War theory. This involves both just cause and just conduct in war. Below I have given its "just cause" criteria.
From what I can see there are three main approaches to the issue of war:
1. Pacifism (never use lethal violence)
2. Just War theory (see below)
3. Holy War (it is right to use lethal violence if we are fighting for God)
I find I can't accept option 3, because the issue comes up: how do we determine if God is on our side? As the scriptures make clear, God is not always on our side. One approach to determine if God is on our side might be to use the Just War criteria - but if you take this approach you might as well say you support the second option. If we use other means to determine if God is on our side, what means are they? Certainly there have been many wars where, according to the warring parties, God has been on both sides.
The Just War criteria below are taken from paragraph 2309 of the Catechism of the Catholic Church (i.e. this is what all Catholics are supposed to believe)
1. the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain
2. all other means of putting an end to it must have been shown to be impractical or ineffective
3. there must be serious prospects of success
4. the use of arms must not produce evils and disorders graver than the evil to be eliminated
The Just War criteria given below are largely taken from Wikipedia. In this case I am happy to use Wikipedia because I have looked into Just War theory before, and this does a reasonably good job of describing the criteria that have traditionally been part of it. After reading it, let me know what you think.
Just War Theory:
Just cause
The reason for going to war needs to be just (which includes self defense and defense of others. It might also include recapturing things taken or punishing people who have done wrong)
Comparative justice
While there may be rights and wrongs on all sides of a conflict, to override the presumption against the use of force, the injustice suffered by one party must significantly outweigh that suffered by the other.
Legitimate authority
Only duly constituted public authorities may wage war.
Right intention
Force may be used only in a truly just cause and solely for that purpose — correcting a suffered wrong is considered a right intention, while material gain or maintaining economies is not.
Probability of success
Arms may not be used in a futile cause or in a case where disproportionate measures are required to achieve success.
Last resort
Force may be used only after all peaceful and viable alternatives have been seriously tried and exhausted or are clearly not practical.
Proportionality
The anticipated benefits of waging a war must be proportionate to its expected evils or harms.
