In 917, a particularly strong Byzantine army led by Leo Phokas the Elder, son of Nikephoros Phokas, invaded Bulgaria accompanied by the Byzantine navy under the command of Romanos Lekapenos, which sailed to the Bulgarian Black Sea ports. En route to Mesembria (Nesebǎr), where they were supposed to be reinforced by troops transported by the navy, Phokas' forces stopped to rest near the river of Acheloos, not far from the port of Anchialos (Pomorie). Once informed of the invasion, Simeon rushed to intercept the Byzantines, and attacked them from the nearby hills while they were resting disorganized. In the Battle of Acheloos of 20 August 917, one of the largest in medieval history, the Bulgarians completely routed the Byzantines and killed many of their commanders, although Phokas managed to escape to Mesembria. Decades later, Leo the Deacon would write that "piles of bones can still be seen today at the river Acheloos, where the fleeing army of the Romans was then infamously slain".

The battle of Achelous was one of the most important battles in the long Byzantine–Bulgarian Wars. It secured the concession of the Imperial title to the Bulgarian rulers, and thereby firmly established Bulgaria's role as a key player in Europe.