Despite his improvements, Alexios did not have enough manpower to recover the lost territories in Asia Minor. Having been impressed by the abilities of the Norman cavalry at Dyrrhachium, he sent ambassadors west to ask for reinforcements from Europe. This mission was deftly accomplished – at the Council of Piacenza in 1095, Pope Urban II was impressed by Alexios's appeal for help, which spoke of the suffering of the Christians of the east and hinted at a possible union of the eastern and western churches.

On 27 November 1095, Urban II called together the Council of Clermont in France. There, amid a crowd of thousands who had come to hear his words, he urged all present to take up arms under the banner of the Cross and launch a holy war to recover Jerusalem and the east from the 'infidel' Muslims. Indulgences were to be granted to all those who took part in the great enterprise. Many promised to carry out the Pope's command, and word of the Crusade soon spread across western Europe. Alexios had anticipated help in the form of mercenary forces from the West, and was totally unprepared for the immense and undisciplined hosts which soon arrived, to his consternation and embarrassment.