The idea that resorting to war can only be just under certain conditions goes back to Cicero and probably beyond, but St. Augustine (Augustine of Hippo) is credited with introducing the theory to Christianity in the fifth century A.D.
The Just War theory has remained a pillar of the Roman Catholic Church as reinforced by "The Challenge of Peace: God's Promise and Our Response," a a 1983 pastoral letter issued by U.S. Conference of Catholic Bishops.
Four strict conditions are listed for "legitimate defense by military force":
* The damage inflicted by the aggressor on the nation or community of nations must be lasting, grave and certain.
* All other means of putting an end to the conflict must have been shown to be impractical or ineffective.
* There must be serious prospects of success.
* The use of arms must not produce evils and disorders graver than the evil to be eliminated.