Walter Ullman. The Growth of Papal Government in the Middle Ages: A study in the ideological relation of clerical to lay power. London: Methuen & Co., 1970. Chapters VIII, "Imperial Hegemony" and IX, "Gregory VII" (pp. 229 309).
A central feature of medieval history was the long struggle for supremacy between the Papacy and the German "Holy Roman" Emperors. In AD 800, the Pope had crowned Charlemagne, King of the Franks, as Roman Emperor, "restoring" the Empire in the West, and establishing by implication the principle that the Papacy was the highest authority in Western Christendom, an authority that could make and, perhaps, by further implication, unmake Roman Emperors.
The Frankish line of Emperors went into abeyance by the end of the century, and their place was taken in the tenth century by a succession of German Emperors, while at the same time the Papacy fell into profound decadence. The German Emperors attempted to establish a regime of "caesaro papism," similar to that of Byzantium, whereby the Emperor was a sacral figure and the supreme head of Church as well as State, with the ability and right to make or depose bishops, including Popes. But, in the conventional interpretation, a wave of reform took hold in the Papacy, culminating with the great figure of Gregory VII, who humbled Emperor Henry IV at Canossa and thus definitively put an end to caesaro papism and established the Papacy itself as the highest fount of moral authority in the Christian West.
Walter Ullman examines the period of imperial ascendency and the reign of Gregory VII in Chapters VIII and IX of his history of papal government and ideology, and demonstrates that the actual course of events was much more complex. Indeed, in Ullman's view, the ideology of the great German Emperors contained within itself the seeds of papal predominance and their own future humbling.
Ullman bases this argument, in...