What factors contributed to Charlemagne being able to call himself an emperor of all peoples? ...

From the earliest point of his reign, Charlemagne intended to form an empire rather than remain simply the king of Franks. He demonstrated this in 774 when he conquered and proclaimed himself king of the Lombards. This was little understood at the time, but Charlemagne had taken the first step toward styling himself as emperor. He was assisted in this by the transition in understanding of the idea of kingship by his father, Pepin, who took control of the Merovingian reign, becoming king rather than mayor of the palace, and then passed the title along to his descendants. Second, at the time Charlemagne began his ascent to power, the Byzantine ruler was the Empress Irene, a woman. Furthermore, the Byzantine position had slipped dramatically in its own crises of iconoclasm and fighting with the spreading Arabic Empire. These factors cast doubt on the legitimacy of the Byzantine claim to the title of emperor. Finally, because Pepin had initiated, and Charlemagne maintained, a close relationship with the pope, in addition to large-scale conversions while conquering lands, there was a general sense of cooperation and establishment of a Christian Empire.