The remains of St Mark had lain in their new shrine in Venice for just over a year when a movement was launched in western Europe which was to have consequences that no one could foresee. In November 1095 Pope Urban II addressed the throng of clerics and laymen assembled for his Council at Clermont. He announced it as his will that the Christians of the west should march to the rescue of their brethren in the east, threatened as they were with extinction by the godless Turks. The whole of a reunited Christendom could then proceed to the liberation of the Holy Land. The idea of an armed pilgrimage, or a crusade, touched a chord in the hearts of his western audience; and as the word got around the response was overwhelming. The pope had proposed that the ‘soldiers of Christ’ should assemble at Constantinople. Within twelve months of the Council at Clermont thousands had set out, trudging overland in more or less disorganised bands. The pope had unwittingly unleashed forces in western society which it would be difficult to control.

The task of controlling them when they reached Constantinople fell on the Emperor Alexios Komnenos. The news of their approach filled him with horror and dismay. Nor was he clear about the pope's intentions. It was not long since the previous pope had denounced him as the excommunicated enemy of western Christendom and the target of a just war directed by that champion of the faith, Robert Guiscard. Since then the emperor had indeed made diplomatic efforts to win the sympathy and support of Pope Urban II, but for a limited purpose.