The theory of the just war (just as in "justice") was developed by St. Augustine and was refined within the Roman Catholic tradition by later writers, including St. Thomas Aquinas and Hugo Grotius.  The theory has been embraced widely within the non-Catholic Western world.  Only just wars were permissible.  In order to be just, a number of conditions had to obtain: 1) The just war must be conducted under the authority of a legitimate ruler. 2) The war must be fought for a just cause.  Typically, that just cause consists of either the defense of one's own country or the protection of an ally.  3) A just war must be conducted in pursuit of some identifiable good, either to punish evil or to preserve or reclaim some identifiable happiness recognized by the world as belonging to the country conducting the just war.  4) The war must be conducted with mercy.  5) Victory must be assured so that the war is not fought merely for national or ethnic pride.