St. Augustine's views on salvation through coercion

This question goes out to any denomination. Why was St. Augustine okay with using force to convert unbelievers to the faith, which also resulted in the eventual Roman Catholic Church's inspiration for the Crusades? As mentioned by Bruce Shelley in his book "Church History in Plain Language": > For Urban and the popes who followed him the Crusades were a new type > of war, a Holy War. Augustine had laid down the principles of a “just > war”: It was conducted by the state; its purpose was the vindication > of justice, meaning the defense of life and property; and its code > called for respect for noncombatants, hostages, and prisoners. All > these evaporated in the heat of the holy cause. Urban appealed for > crusaders in the name of the church; the purpose of the Crusades was > the conquest of the infidels in the Holy Land; and this high calling > excused somehow the total disregard for noncombatants and prisoners. Also per this link : > Urban pulled ideas from St. Augustine to justify the calling of a holy > war to get rid of enemies of Christianity, and Urban had Pope Gregory > VII to thank for already having a group loyal to the papacy so Urban’s > work on power was already partially done (Asbridge, 25-28). Asbridge > points out that “A central feature of Urban’s doctrine was the > denigration and dehumanisation of Islam (33).” As a follow up what caused him to be for that strategy when it came to conversion?