The battle of Achelous was led at the coast of Black sea on 20th of August 917 between the Bulgarian troops, led by king Simeon I; and the Byzantine troops. This was one of the biggest battles of the whole middle ages with more than 120 000 participants. The main aim of the battle is suppressing of the rising new Bulgarian state by the Byzantines. The result of the battle was total Bulgarian victory, which made the Bulgarian king Simeon I ruler of the whole Balkan peninsula, except the well protected fortresses Constantinople and Peloponnese.
The battle of Achelous was the worst ever defeat of the Byzantine army, and one of the biggest military successes of Bulgaria ever.
In the beginning of the battle the Byzantine troops have advance. There has been a hard clash in the middle of the battlefield, after what the Bulgarians become to retreat in the center and the left wing. The Byzantine army, feeling victory, attacks massively even with the reserve troops, but destroy the formation. In this critical moment king Simeon I counterattacks with heavy armed horsemen and hidden reserves. The surprised and disorientated Byzantine soldiers become retreat to the sea, unable to resist seriously. The Bulgarian troops start total massacre, those of the Byzantines who are not slaughtered get drown into Black sea or the river Anchialus. Only 2000 Byzantine horsemen succeed to escape on South.
This huge loose of men blocks the military power of the Byzantine empire for the next years, and leads to crisis and cataclysms. On the other side, Bulgaria at this time is on the top of its glory and majesty, and the period is known as The Golden Age.