Henry Davis: Medieval Europe. Williams and Norgate, London 1911, p. 55: "These crowded years of war leave the Frankish Empire established as the one great power west of the Elbe and Adriatic."
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Daniel Ziemann: Das Erste bulgarische Reich. Eine frühmittelalterliche Großmacht zwischen Byzanz und Abendland. (German: An early medieval great power between Byzantium and the Occident) In: Online handbook on the history of South-East Europe. Volume I Rule and politics in Southeastern Europe until 1800. Published by the Institute for East and Southeast European Studies of the Leibniz Association, Regensburg 2016
