How a betrayed alliance led to a crushing Byzantine defeat at the Black Sea
In 708, the Byzantine Emperor Justinian II attempted to reclaim territory he had previously gifted to the Bulgarian Khan Tervel. The resulting clash at Anchialus turned into a tactical disaster for the Byzantines, leaving their emperor to flee by sea while his army was decimated by a surprise Bulgarian assault.
The conflict originated from a fragile political debt. In 705, Khan Tervel had provided critical military support to help Justinian II regain the Byzantine throne after a decade of exile. In appreciation, Justinian granted the Bulgarians the Zagore region, a strategic area spanning from Stara Zagora and Sliven to the Black Sea. However, by 708, Justinian felt sufficiently secure in his power to attempt a reclamation of these lands, leading to an invasion that would test the strength of the Bulgarian state.
The Byzantine military strategy faltered due to a lack of intelligence. Upon arriving at the fortress of Anchialus, near the modern-day town of Pomorie, the Byzantine forces established a camp, completely unaware that the Bulgarian army was positioned nearby. As the invaders focused on gathering provisions, Tervel launched a coordinated two-pronged offensive. His cavalry struck the outer perimeter of the Byzantine troops, while his infantry simultaneously stormed the main camp. The suddenness of the assault caused widespread confusion, leading to the capture or death of a vast number of Byzantine soldiers, along with the loss of significant quantities of arms and horses.
The aftermath of the battle solidified Bulgarian control over the disputed region for centuries. Justinian II was among the few who managed to reach the safety of the fortress, eventually escaping back to Constantinople by ship. The political fallout continued until 711, when a coup forced the emperor to once again seek aid from Tervel. Despite their previous hostility, Tervel provided 3,000 soldiers to support him. Ultimately, these troops were granted safe passage home by the new Byzantine leadership after Justinian II was executed, marking a definitive end to his tumultuous reign and the stabilization of the Bulgarian border.
Source: Battle of Anchialus (708)
