Fearing the potential conversion of Boris I, khan of the Bulgars to Christianity under Frankish influence, Michael III and the Caesar Bardas invaded Bulgaria, imposing the conversion of Boris according to the Byzantine rite as part of the peace settlement in 864. Michael III stood as sponsor, by proxy, for Boris at his baptism. Boris took the additional name of Michael at the ceremony. The Byzantines also allowed the Bulgarians to reclaim the contested border region of Zagora. The conversion of the Bulgarians has been evaluated as one of the greatest cultural and political achievements of the Byzantine Empire.