The Byzantine army of emperor Constantine V defeats the Bulgarian forces in the Battle of Anchialus.

The Byzantine Empire, also known as the Eastern Roman Empire, was the continuation of the Roman Empire centred on Constantinople during late antiquity and the Middle Ages. Having survived the events that caused the fall of the Western Roman Empire in the 5th century AD, it endured until the fall of Constantinople to the Ottoman Empire in 1453. The term 'Byzantine Empire' was coined only after its demise; its citizens used the term 'Roman Empire' and called themselves 'Romans'.

Constantine V was Byzantine emperor from 741 to 775. His reign saw a consolidation of Byzantine security from external threats. As an able military leader, Constantine took advantage of civil war in the Muslim world to make limited offensives on the Arab frontier. With this eastern frontier secure, he undertook repeated campaigns against the Bulgars in the Balkans. His military activity, and policy of settling Christian populations from the Arab frontier in Thrace, made Byzantium's hold on its Balkan territories more secure. He was also responsible for important military and administrative innovations and reforms.

The First Bulgarian Empire was a medieval state that existed in Southeastern Europe between the 7th and 11th centuries AD. It was founded in 680–681 after part of the Bulgars, led by Asparuh, moved south to the northeastern Balkans. There they secured Byzantine recognition of their right to settle south of the Danube by defeating – possibly with the help of local South Slavic tribes – the Byzantine army led by Constantine IV. During the 9th and 10th century, Bulgaria at the height of its power spread from the Danube Bend to the Black Sea and from the Dnieper River to the Adriatic Sea and became an important power in the region competing with the Byzantine Empire.

The battle of Anchialus occurred in 763, near the town of Pomorie on the Bulgarian Black Sea Coast. The result was a Byzantine victory.