The Battle of Achelous

Bulgaria's steadily growing geopolitical importance in the Balkans caused deep concern in Byzantium. Constantinople decided to halt all other military operations and direct every available force against Bulgaria with the aim of destroying it. A Byzantine army of more than sixty thousand invaded Bulgarian territory, advancing along the Black Sea coast. On 20 August 917, near Anchialos and the Achelous River, it confronted the Bulgarian army commanded by Tsar Simeon. The Byzantines' initial advantage was overcome when Simeon committed his heavy cavalry and infantry reserves. Pressed against the sea and river, the Byzantine formation fell into complete chaos, and its troops fled the battlefield, unable to offer resistance.

The battle ended in success for the Bulgarian state and Tsar Simeon. The crushing defeat caused internal political upheaval in Byzantium and gave Bulgaria strong grounds for its ambition to dominate the region.