Complex sentence: After the beginning of the Anti-Byzantine rebellion in 1185 and especially after the victory at Tryavna the Bulgarians firmly took the initiative of the war started recapturing regions of Thrace and Macedonia from their strongholds and bases to the north of the Balkan mountains and the Danube river.
Split into simpler sentences: After the beginning of the Anti-Byzantine rebellion in 1185 and especially after the victory at Tryavna the Bulgarians firmly took the initiative of the war. As a result of their efforts, they started capturing regions of Thrace and Macedonia from Byzantines, and removed the Byzantine strongholds and bases to the north of the Balkan mountains and the Danube river.