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In page Konstantin Tih:

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Mitso, who still held southeastern Bulgaria, made an alliance with the Byzantines, but another powerful nobleman, Jacob Svetoslav, who had taken control of the southwestern region, was loyal to Konstantine.[5] Benefiting from a war between the Byzantine Empire, the Republic of Venice, Achaea and Epirus, Konstantine invaded Thrace and captured Stanimaka and Philippopolis (now Asenovgrad and Plovdiv in Bulgaria) in the autumn of 1262.[5][6] Mitso was also forced to flee to Mesembria (now Nesebar in Bulgaria).[7] After Konstantine laid siege to the town, Mitso sought assistance from the Byzantines, offering to surrender Mesembria to them in exchange for landed property in the Byzantine Empire.[8] Michael VIII accepted the offer and sent Michael Glabas Tarchaneiotes to help Mitso in 1263.[7]