 Complete the following phrase:  On the death of Simeon in May 927, Bulgarias new emperor, Peter I, made a show of force by invading Byzantine Thrace, but he showed himself ready to negotiate for a more permanent peace. Romanos seized the occasion and proposed a marriage alliance between the imperial houses of Byzantium and Bulgaria, at the same time renewing the Serbian-Byzantine alliance with Časlav of Serbia, returning independence the same year. In September 927 Peter arrived before Constantinople and married Maria renamed Eirene, Peace, the daughter of his eldest son and co-emperor Christopher, and thus Romanos granddaughter. On this occasion Christopher received precedence in rank over his brother-in-law Constantine VII, something which compounded the latters resentment towards the Lekapenoi, the Bulgarians, and imperial marriages to outsiders as documented in his composition De Administrando Imperio. From this point on, Romanos government was free from direct military confrontation with Bulgaria. Although Byzantium would tacitly support a Serbian revolt against Bulgaria in 931, and the Bulgarians would allow Magyar raids across their territory into Byzantine possessions, Byzantium and Bulgaria remained at peace for 40 years, until 