The difference between Augustinian “just war” and “crusade”:

The standard for a Christian “just war” as developed by Augustine (c. A.D. 400) is: “rightful intention on the part of the participants, which should always be expressed through love of God and neighbour; a just cause; and legitimate proclamation by a qualified authority.” (Quoted from J. Riley-Smith, The Crusades, Yale University, 1987.)  The doctrine of holy war/crusade added two further assumptions: 1) Violence and its consequences–death and injury–are morally neutral rather than intrinsically evil, and whether violence is good or bad is a matter of intention. (The analogy is to a surgeon, who cuts into the body, thus injuring it, in order to make it better/healthier.)  2) Christ is concerned with the political order of man, and intends for his agents on earth, kings, popes, bishops, to establish on earth a Christian Republic that was a “single, universal, transcendental state’ ruled by Christ through the lay and clerical magistrates he endowed with authority.

It follows from this that the defense of the Christian Republic against God’s enemies, whether foreign infidel (e.g. Turks) or domestic heretics and Jews was a moral imperative for those qualified to fight. A Crusade was a holy war fought against external or internal enemies for the recovery of Christian property or defense of the Church or the Christian people. It could be wages against Turks in Palestine, Muslims in Spain, pagan Slavs in the Baltic, or heretics in southern France, all of whom were enemies or rebels against God. https://www.youtube.com/user/RealCrusadeHistory