(1) The choice to go to war
Just cause. The right to self-defense against an aggressor has consistently been regarded as fundamental. Only defensive war is legitimate.
Last resort. War may be waged only when all negotiations and compromise have been attempted and have failed. In his “Commentary on the Sermon on the Mount,” Luther remarks that “anyone who claims to be a Christian and a child of God, not only does not start war or unrest; also he gives help and counsel on the side of peace wherever he can, even though there be a just and adequate cause for going to war.”
Formal declaration. Since the use of military force is the prerogative of government and not of private individuals, properly constituted procedure for declaring and waging war must be followed. As Thomas Aquinas commented, “it is not the business of a private individual to declare war ... as the care of the common weal is committed to those who are in authority.”
Just Intention. War must be carried out to secure a just peace, not for territorial conquest, economic gain, or ideological supremacy. The only legitimate intention of war is to secure peace.
(2) The proper actions during war
The principle of proportionality. The weaponry and force used should be limited to what is needed to secure a just peace and attain better conditions after the conflict than existed prior to it.
The principle of discrimination. Since war is an official act of government, noncombatants and civilians should be immune from attack.
The principle of limited objectives. Since the purpose of a just war must ultimately be peace, unconditional surrender or the complete removal of the social or political institutions of a nation is unwarranted.
Resource: John F. Johnson, "Can War Be Just?"