The Holy Roman Empire, a revival of the ancient Imperial Roman state, was founded at the outset of the 9th century by Charlemagne, who in 800 had himself crowned Holy Roman Emperor by Pope Leo III in Rome.
Later, between the13th and 15th centuries, the emperors came to rule over a large federation of German principalities. The emperors regarded themselves as co-regents with the papacy and traced the legitimacy of their position not only back to Charlemagne, but also to Constantine, the first imperial Roman emperor to embrace Christianity.
See the entry for Charles V.