Editing Imperium (section)

== Later Roman Empire ==
While the Byzantine [[Byzantine Emperor|Eastern Roman Emperors]] retained full Roman ''imperium'' and made the episcopate subservient, in the feudal West a long rivalry would oppose the claims to supremacy within post-Roman Christianity between ''sacerdotium'' in the person of the Pope and the secular ''imperium'' of the Holy Roman Emperor, beginning with Charlemagne, whose title was claimed to have "restored" the office of Western Roman Emperor among the new kingdoms of Western Europe. Both would refer to the heritage of Roman law by their titular link with the very city of Rome: the Pope, Bishop ''of Rome'', versus the Holy ''Roman'' Emperor (even though his seat of power was north of the Alps).

The ''[[Donation of Constantine|Donatio Constantini]]'', by which the Papacy had allegedly been granted the territorial [[Patrimonium Petri]] in Central Italy, became a weapon against the Emperor. The first pope who used it in an official act and relied upon it, [[Leo IX]], cites the "Donatio" in a letter of 1054 to [[Michael Cærularius]], Patriarch of Constantinople, to show that the Holy See possessed both an earthly and a heavenly ''imperium'', the royal priesthood. Thenceforth, the "Donatio" acquires more importance and is more frequently used as evidence in the ecclesiastical and political conflicts between the papacy and the secular power: [[Anselm of Lucca]] and [[Deusdedit of San Pietro in Vincoli|Cardinal Deusdedit]] inserted it in their collections of canons; [[Gratian]] excluded it from his ''Decretum'', but it was soon added to it as ''Palea''; the ecclesiastical writers in defence of the papacy during the conflicts of the early part of the 12th century quoted it as authoritative.

In one bitter episode, [[Pope Gregory IX]], who had several times mediated between the Lombards and the [[Holy Roman Emperor]] [[Frederick II, Holy Roman Emperor|Frederick II]], reasserted his right to arbitrate between the contending parties. In the numerous manifestos of the Pope and the Emperor the antagonism between Church and State became more evident: the Pope claimed for himself the ''imperium animarum'' ("command of the souls", i.e. voicing God's will to the faithful) and the ''principatus rerum et corporum in universo mundo'' ("primacy over all things and bodies in the whole world"), while the Emperor wished to restore the ''imperium mundi'', ''imperium'' (as under Roman Law) over the (now Christian) world. Rome was again to be the capital of the world and Frederick was to become the real emperor of the Romans, so he energetically protested against the authority of the Pope. The emperor's successes, especially his victory over the Lombards at the [[battle of Cortenuova]] (1237), only aggravated tensions between Church and State. The pope again excommunicated the "self-confessed heretic", the "blasphemous beast of the Apocalypse" (20 March 1239) who now attempted to conquer the rest of Italy (i.e., the [[papal states]], et cetera).