Up to this period the See of Rome had gone far toward establishing an ecclesiastical monarchy. Every principle of an unlimited religious autocracy had been asserted and to a considerable extent established. The outward machinery for this spiritual absolutism had been created and partially put in motion. But many obstacles to the smooth working of the system were still encountered. Chief among these impediments was the strong arm of the eastern Empire. Until the fetters of political dependence were broken, the Papacy could never accomplish its great mission. Hitherto the Church of Rome had assumed a political headship on many occasions, but it was the result of some accidental emergency and soon disappeared. Nevertheless the experience gained in this exercise of secular authority created an ambition on the part of the Roman Pontiffs for political independence, furnished precedents for future claims, and led the Italians to believe that the head of the Church could give them efficient government in temporal affairs as well as spiritual. The great problem before the successors of St. Peter at this time was how to manage the ecclesiastical ascendency already gained over the Western Church, so as to render it serviceable in securing that political self-existence so essential not only to maintain the ground already won but also to realise their high hopes in other directions. At this juncture a combination of external causes, unparalleled in the world's history, came in to favour the emancipation of the Papacy from the last feeble bonds of a nominal dependency and to permit of the assumption of temporal sovereignty virtually if not in recognised title. This meant the realisation of the mediæval Church. Emperor Leo's attempt to abolish the worship of images in Christendom provoked a rebellion in Italy headed by the Pope. Luitprand, seeing his opportunity as King of the Lombards, fell on the exarchate as the champion of images and on Rome as the supposed ally of the Emperor. The Pope, perilously placed between a heretic and an invader, appealed for help to a Catholic chief across the Alps who had just saved Christendom by defeating the Mohammedans on the field of Poitiers. Gregory III. excommunicated the eastern Emperor and begged Charles Martel to hasten to the succour of the Holy Church. Here the Roman Pontiff leads a political revolt against his legitimate sovereign and appeals to a foreign power to make the revolt successful. The Bishop of Rome has stepped into the position of a temporal prince with the political future of Italy in his hands.