The capture of Jerusalem on 15 July 1099 was of enormous importance in the history of the Christian West and the Orthodox and Islamic East. It was the culmination of five years of incredible effort which began in November 1095 when Pope Urban II launched the great expedition at Clermont in the Auvergne. For those who laboured on it and survived, the seizure of Jerusalem represented a triumphant consummation. Little wonder that amongst the appalling carnage and slaughter of the sack, in which men ‘rode in blood up to their knees and bridle reins’, they believed ‘it was a just and splendid judgment of God that this place should be filled with the blood of the unbelievers, since it had suffered so long from their blasphemies’ and rejoiced. ‘A new day, new joy, new and perpetual gladness, the consummation of our labour and devotion, drew forth from all new words and new songs’. For the papacy, which conceived the idea of the expedition, its victory was a vindication of the pope's claims to be the leader of the Christian world. In the ideological conflict between empire and papacy, which we call the ‘Investiture Contest’, the victory of the crusade tipped the balance sharply towards the papacy.