Simeon I of Bulgaria
The size of the Bulgarian army under Simeon I of Bulgaria is unknown. Although they ruined the Byzantine negotiations, the Bulgarians were still afraid that the old allies of the Byzantines, the Pechenegs and the Hungarians, would attack them from the north, so two small armies were sent to protect the northern borders of the vast Bulgarian Empire that spread from Bosnia in the west to the Dnieper River in the east. However, "Miracula Sancti Georgii" points that the Bulgarian army in the battle of Achelous was allied with Hungarian and Pecheneg troops, which helped to win the victory against the Byzantine army. In addition, Bulgarian forces under Marmais were deployed near the western borders with the Serb principalities to prevent possible unrest.