Simeon I of Bulgaria
The Byzantine–Bulgarian Wars made the First Bulgarian Empire "de facto" the most powerful Empire of Southeastern Europe. The Bulgarians won after invading at the right time; they met little resistance in the north because of the Byzantines fighting the Arabs in Anatolia but eventually the Byzantines concluded peace with the Arabs. They sent their whole army against the Bulgarians but were decisively defeated in the battle of Achelous in 917. After several more victories Simeon I of Bulgaria prevailed. Petar switched sides to the Byzantines and was deposed and sent to Bulgaria; Pavle Branović was instated by the Bulgarians in 917, ruling until 920–921.