 Complete the following phrase:  During the reign of Emperor Peter I, Bulgaria prospered in a long-lasting peace with Byzantium. This was secured by the marriage of Peter with the Byzantine princess Maria Lakapina, granddaughter of Byzantine Emperor Romanos I Lekapenos. However, after Marias death in 963, the truce had been shaken and it was at this time or later that Peter I sent his sons Boris and Roman to Constantinople as honorary hostages, to honor the new terms of the peace treaty. During these years the Byzantines and Bulgarians had entangled themselves in 