Gregory was an ambitious prelate, but he was also fighting for a principle that was higher than mere ambition. The right of investiture, which amounted to a claim on the part of the emperor to choose bishops, was an effective barrier to the reform of the Church, and the same obstructionism was encountered everywhere where kings for their own ends protected refractory ecclesiastics or suppressed rebellious ones. Gregory boldly challenged the Emperor's claim to domination over bishops as their secular overlord. This phase of the Investiture dispute, as it is called, ended in the humiliation of the Emperor Henry IV. at Canossa, where he humbly submitted to Gregory in 1077. But the fight was not over. It ended in wars and alarms, till finally Gregory abandoned it to die in exile. The compromise of 1122 settled the immediate issue, but virtually it was a defeat for the Empire, and so for the whole feudal system. The system was turned against the temporal upholders of it, for the overlord-ship was now interpreted as giving the Pope what was virtually a feudal overlordship over the secular monarch, a claim that was completely destructive of the system as it had evolved.

The prestige of the system was likewise shaken by theCrusades, that most extraordinary of all the episodes of the Middle Ages. Their occurrence, though not their form, was dictated by the evolution of Western civilisation. Despite all its feudal wars it was developing the arts of peace at an amazing pace, and in trade particularly it felt the need of expansion. Already its emissaries were finding new fields, and the ports of the North Sea and the Mediterranean were sending their fleets into new seas. In the East the existence of a fanatical anti-Christian power, divided though it was into many kingdoms, was an insuperable barrier. The Seljuk Turks had been pressing westward from Central Asia for centuries, and in the middle of the eleventh century dominated the eastern Caliphate of Baghdad. They had accepted Mohammedanism, but were not patrons of culture such as the old Caliphs had been, and their rule opposed to the West a buttress as formidable as it was irksome. The banning of the pilgrim from the Holy Places, and the claims of trade as well as of religion combined to call upon the West for a united effort to clear a way. It is true that in the minds of the Popes who preached the Crusades and failed to organise them, there was present the noble idea of causing civil wars in Christendom to cease by combining its forces against the infidel, and the ignoble one of giving the Papacy headship over it. But the aim was hopeless of realisation from the first. National evolution had proceeded too far, and the consciousness of a deeper unity than that of mere allegiance to a dominant Church was yet to arise.