"Emperor Honorius decreed that heretics and pagan worshipers were to be punished by "exile and blood." [Ellerbe, 28]
Repression of pagans by secular authorities was "unavoidable" and "not necessarily a case of persecution for religious opinions." [Catholic Encyclopedia, "Donatists"]
After talks had failed, [St.] Augustine of Hippo (in Africa), reversed his long-standing position and sanctioned the use of force against the Donatists. He promulgated the principle Cognite intrare, 'Compel them to enter.' The church would use this doctrine time and again to justify intolerance and violent repression of dissent, heresy, and other religions. [Ellerbe, 37-38]
Augustine based his Cognite intrare principle on Luke 14:23: "The master then ordered the servant, 'Go out to the highways and hedgerows and make people come in that my home may be filled.'" This verse was part of a parable about a man who wanted to give a great dinner and his invited guests made excuses not to come. Somehow it became an excuse to force people to convert to Christianity and to punish them if they refused.
The doctrine of a "holy war" or "just war" was developed by Augustine. He said that a defensive war is always just and that an offensive war is just when waged against "a state that refuses to make reparations for wrongs committed or fails to return seized property." [Williams, 2002, 29]"