When, after more than a century, the fourth uprising did take place in 1186, it was under the auspices of two local chieftains, Peter and Asen, whom all the sources, Byzantine and western, agree in calling Vlachs, and who lived in the Balkan mountains. [37] The most elaborate theories have been resorted to by Bulgarian historians to prove that the word 'Vlach' had by 1185 come to mean a 'Bulgar from the northwest part of the country.' They explain this phenomenon as the result of a conspiracy of silence on the part of Byzantine writers, who, they argue, were trying to avoid the use of the word 'Bulgar' and to substitute 'Vlach.'
In fact, however, it can be demonstrated that Byzantine writers use the word 'Bulgar' quite freely when they are talking about Bulgars, and use 'Vlach' only to refer to Vlachs. It has long been realized that any other interpretation involves the twisting of the sources until they bear no relationship to the ascertainable facts about the origin and development of the ‘second Bulgarian Empire.’
In the next year, the forces of Frederick Barbarossa, moving on Constantinople, presented a grave threat to the Empire. As they crossed the Balkans in July 1189, they received letters from Peter, who had joined with two Serb zhupans in an alliance against Isaac. The allies now offered Frederick aid in any operation he might undertake against Byzantium. The offer was renewed that winter, when Peter specifically promised Frederick 40,000 Vlach and Cuman archers for an attack on Constantinople, scheduled for the beginning of spring 1190; and this offer Frederick was tempted to accept. But he reached an agreement with Isaac during the winter, and decided not to attack Constantinople. Isaac's 'dapifer magnus' asked Barbarossa for aid against the Vlachs, and, on the same day, a representative of Peter the Vlach arrived, eager to get Frederick's aid against the Greeks. Frederick refused both requests. This is almost all we know of the relations between Barbarossa and the Vlach brothers.