His complex theories were later simplified to produce just three prerequisites of a Just War: proclamation by a ‘legitimate authority’, such as a king or bishop; a ‘just cause’, like defence against enemy attack or the recovery of lost territory; and prosecution with ‘right intention’, that is, with the least possible violence. These three Augustinian principles underpinned the crusading ideal, but they fell far short of advocating the sanctification of war.
These "complex theories" were from St. Augustine of Hippo (354–430 CE) who came several centuries before the Crusades. The Crusades was instigated by Pope Urban in 1095.
