The main source on the restoration of the Second Bulgarian Empire is the Byzantine historian
Nicetas Choniates. Choniates refers to the people of Peter and Ivan Asen as "the
barbarians around Mount
Haimos, who were earlier called
Mysoi, and are now called Blachoi" (Choniates, 482 [p. 368 van Dieten]). The designation "Mysians" is derived from the Roman province of
Moesia, corresponding to the territory between the Balkan (Haimos/Haemus) mountains and the Lower
Danube.
The term Mysians had been used to designate the Bulgarians by classicizing Byzantine historians since at least Leo the Deacon in the second half of the 10th century. (In the same classicizing vein, Byzantine authors were want to call the
Russians "
Scythians" and the
Serbs "Triballoi".) To add to the confusion, elsewhere in Choniates' history, the subjects of Peter IV and Ivan Asen I are on occasion called "Mysians", "Vlachs" or two different but conjoined peoples, "the people [
génos] of the Bulgarians and Vlachs" (Choniates, 485 [p. 371 van Dieten]). The contemporary German (Austrian) chronicler
Ansbert mentions "the Vlach Kalopetrus and his brother Assanius" (33), and calls Peter
Blacorum et maxime partis Bulgarorum in hortis Tracie dominus, "ruler of the Vlachs and the greatest part of the Bulgarians in the gardens of
Thrace" (58).
The eminent Bulgarian historian
Vasil Zlatarski has drawn attention to the fact that under Byzantine rule Bulgaria proper was divided between a theme of Bulgaria (in the west) and a theme called Paradounabion/Paristrion and later Moesia (in the east). Since Niketas Choniates explicitly states that "the Mysians ... are now called Vlachs", Zlatarski concludes that the conjoint terms Bulgarians and Vlachs found in the sources indicate the extension of Peter IV and Ivan Asen I's control over the population of both themes, Bulgaria and Moesia. This conclusion is supported by the testimony of Ansbert, who would be correct to identify Peter IV as master of (all) Moesia (as ruler of the Vlachs) and of (a part) of Bulgaria (as ruler of the greater part--superlative!--of the Bulgarians). However, Zlatarski's analysis glosses over the important implication that in order for the Mysians to be called Vlachs in Choniates' time, there must have been very significant Vlach (Wallachian) population on the territory of Moesia itself. This means that even if the medieval description of the population is based primarily on the administrative division of the themes, the popular support for the rebellion of Peter IV and Ivan Asen I consisted of both Bulgarians and Vlachs, rather than exclusively one group or the other.