On 20 August 917, the battle between the Romans and Bulgarians was fought by the river Acheloos, 8 kilometers north from Anchialos on Bulgaria's Black Sea coast. The Romans were completely routed. Their headlong flight was punctuated by fearful cries as some men were trampled by comrades and others were killed by the enemy; there was such a letting of blood as had not been seen for very many years. Leo Phokas was saved by fleeing to Mesembria in Bulgaria, but in the thick of the battle Constantine Lips, John Grapson and many other commanders (archontes) were cut down.
The battle was fought furiously. The decisive moment came when a heavy cavalry corps of Bulgarians, led by Simeon I himself, attacked the Byzantine left wing from behind the hills. It is estimated that approximately 70,000 Byzantine soldiers died in this battle. The Byzantine historian Paulus Deakon says that 75 years after this military catastrophe the field at Anchialus was still covered with tens of thousands of Roman skeletons.
The remainder of the Byzantine army fled all the way back to Constantinople, followed by the Bulgarians, who defeated John again outside the city. The Byzantines proposed a new peace treaty, and Simeon entered the imperial city and was crowned for a second time as "Tsar" (the Slavonic title for Caesar) "of all Bulgarians and Romans".