In 917, Leo Phokas was placed in charge of a large-scale expedition against the Bulgarians. The plan involved a two-pronged assault, one from the south by the main Byzantine army under Leo Phokas, and one from the north by the Pechenegs, who were to be ferried across the Danube by the Byzantine navy under Romanos Lekapenos. In the event, however, the Pechenegs did not help the Byzantines, partly because Lekapenos quarrelled with their leader (or, as Runciman suggests, might have even been bribed by the Bulgarians) and partly because they had already begun plundering on their own, disregarding the Byzantine plan. Left unsupported by both the Pechenegs and the fleet, Phokas suffered a crushing defeat at the hands of Tsar Symeon at the Battle of Acheloos. 

The Battle of Achelous, also known as the Battle of Anchialus, took place on 20 August 917, on the Achelous river near the Bulgarian Black Sea coast, close to the fortress Tuthom (modern Pomorie) between Bulgarian and Byzantine forces. The Bulgarians obtained a decisive victory which not only secured the previous successes of Simeon I, but made him de facto ruler of the whole Balkan Peninsula, excluding the well-protected Byzantine capital Constantinople and the Peloponnese. The battle, which was one of the biggest and bloodiest battles of the European Middle Ages, was one of the worst disasters ever to befall a Byzantine army, and conversely one of the greatest military successes of Bulgaria. Among the most significant consequences was the official recognition of the imperial title of the Bulgarian monarchs, and the consequent affirmation of Bulgarian equality vis-à-vis Byzantium.