Knights, Jerusalem, the Seljuk Turks - the history of the Crusades and the future of Islam, by Rubinstein in The New English Review. One quote:
The knights who entered combat against Muslims and pagans did so for Christian objectives. This led to a radical change in Christian thinking about war. Before the Crusades, the Church was ambivalent about killing and wounding in war. St. Augustine helped to formulate the idea of a “just war" by arguing that killing carried out in obedience to divine command or legitimate public authority did not violate the commandment "Thou shalt not kill." Nevertheless, even a “just war” did not receive the Church’s wholehearted approval. A war that the Church promoted and blessed, such as the First Crusade, had been unthinkable.
Pope Gregory VII (1073-85). was the prelate most responsible for the Church’s changed attitude towards war. One of his favorite quotations from Scripture was “Cursed be he who keepeth back his sword from blood.” (Jeremiah 48:10). Gregory proclaimed a new type of Christian soldier-saint. Before Gregory, Saint George and Saint Sebastian had become saints in spite of having been soldiers. As a result of his teachings, men became saints because they had been soldiers. The long-range consequences of this transformation remain with us to this day.