The Battle of Achelous or Acheloos (Bulgarian: Битката при Ахелой, Greek: Μάχη του Αχελώου), 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 a ruler of the whole Balkan Peninsula excluding the well-protected Byzantine capital Constantinople and the Peloponnese.