Battle of Achelous (917) in the context of Battle of Katasyrtai

The Byzantine–Bulgarian war of 913–927 (Bulgarian: Българо–византийска война от 913–927) was fought between the Bulgarian Empire and the Byzantine Empire for more than a decade. Although the war was provoked by the Byzantine emperor Alexander's decision to discontinue paying an annual tribute to Bulgaria, the military and ideological initiative was held by Simeon I of Bulgaria, who demanded to be recognized as Tsar and made it clear that he aimed to conquer not only Constantinople but the rest of the Byzantine Empire, as well.

In 917, the Bulgarian army dealt a crushing defeat to the Byzantines at the Battle of Achelous, resulting in Bulgaria's total military supremacy in the Balkans. The Bulgarians again defeated the Byzantines at Katasyrtai in 917, Pegae in 921 and Constantinople in 922. The Bulgarians also captured the important city of Adrianople in Thrace and seized the capital of the Theme of Hellas, Thebes, deep in southern Greece. Following the disaster at Achelous, Byzantine diplomacy incited the Principality of Serbia to attack Bulgaria from the west, but this assault was easily contained. In 924, the Serbs ambushed and defeated a small Bulgarian army on its way to Serbia, provoking a major retaliatory campaign that ended with Bulgaria's annexation of Serbia at the end of that year.

The Bulgarian–Serbian wars of 917–924 (Bulgarian: Българо–сръбски войни от 917–924) were a series of conflicts fought between the Bulgarian Empire and the Principality of Serbia as a part of the greater Byzantine–Bulgarian war of 913–927. After the Byzantine army was annihilated by the Bulgarians in the battle of Achelous, the Byzantine diplomacy incited the Principality of Serbia to attack Bulgaria from the west. The Bulgarians dealt with that threat and replaced the Serbian prince with a protégé of their own. In the following years the two empires competed for control over Serbia. In 924 the Serbs rose again, ambushed and defeated a small Bulgarian army. That turn of events provoked a major retaliatory campaign that ended with the annexation of Serbia in the end of the same year.

The Aheloy (Bulgarian: Ахелой), also known as the Achelous, is a river in eastern Bulgaria. It is 40 km long. The river is famous for being the site of the Battle of Achelous that took place on 20 August 917 between Bulgarian ruler Simeon I and the Byzantines under Leo Phocas during the Byzantine–Bulgarian war of 913–927. It was one of the largest battles in the Middle Ages and among the greatest military successes of the First Bulgarian Empire.

The main stem is the Arnautska reka, which springs from the Aytoska Planina division of the eastern Balkan Mountains east of the village of Dryankovets. The river proper is formed from the confluence of the Arnautska reka with the Mangarska reka at an altitude of 166 m, about three kilometers east of the village of Belodol. The Aheloy flows eastwards in a wide alluvial valley until the village of Aleksandrovo, where it turns southeast for the remainder of its length. It flows into the Gulf of Burgas of the Black Sea near a campsite some 1.2 km south of the town of Aheloy.