The Battle of Achelous or Acheloos (, ), 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. Background After the Bulgarian victory in the war of 894–896, the Byzantines were forced to pay tribute to Tsar Simeon I of Bulgaria. In 912 when the Byzantine emperor Leo VI died, his brother Alexander refused to pay tribute to the Bulgarians. Simeon saw an opportunity to wage a new war and fulfil his ambitions to conquer Constantinople. Alexander died in the same year and the new government under the Patriarch Nicholas Mystikos made desperate attempts to avoid the war, promising that the infant Emperor Constantine VII would marry one of Simeon's daughters. At some point, the patriarch and Simeon even met outside the walls of Constantinople, performing a coronation ceremony. Thereafter, Simeon began using the title "Tsar of the Bulgarians", and the Greek title basileus in his seals. After a plot in the Byzantine court in 914, however, the new regent Zoe, Constantine's mother, rejected the marriage. In answer the Bulgarians raided eastern Thrace. Adrianople opened its gates to Simeon in September 914, and its population recognised Simeon as their ruler, while the Byzantine army was occupied in the east. In the next year the Bulgarian armies attacked the areas of Dyrrhachium and Thessalonica. Preparations for battle Both sides carefully prepared for a decisive end of the conflict. Empress Zoe wanted to swiftly make a peace settlement with the Arabs and to engage the whole army of the East in a war with Simeon and destroy him. The Byzantines tried to find allies and sent emissaries to the Magyars, Pechenegs and Serbs, but Simeon was familiar