A conflict between the Byzantines and Bulgarian Empire occurred during 855 and 856. The Byzantine Empire wanted to regain its control over some areas of Thrace, including Philippopolis (Plovdiv) and the ports around the Gulf of Burgas on the Black Sea. Byzantine forces, led by the emperor and the caesar Bardas, were successful in reconquering a number of cities – Philippopolis, Develtus, Anchialus and Mesembria among them – as well as the region of Zagora. At the time of this campaign the Bulgarians were distracted by a war with the Franks under Louis the German and the Croatians. In 853 Boris had allied himself to Rastislav of Moravia against the Franks. The Bulgarians were heavily defeated by the Franks; following this, the Moravians changed sides and the Bulgarians then faced threats from Moravia.