In 680, the Bulgars under Khan Asparukh crossed the Danube into nominally Imperial territory and began to subjugate the local communities and Slavic tribes. In 680, Constantine IV led a combined land and sea operation against the invaders and besieged their fortified camp in Dobruja. Suffering from bad health, the Emperor had to leave the army, which panicked and;was defeated;at the hands of Asparuh at Onglos, a swampy region in or around the Danube Delta where the Bulgars had set a fortified camp. The Bulgars advanced south, crossed the Balkan Mountains and invaded Thrace.

In 681, the Byzantines were compelled to sign a humiliating peace treaty, forcing them to acknowledge Bulgaria as an independent state, to cede the territories to the north of the Balkan Mountains and to pay an annual tribute. In his universal chronicle the Western European author Sigebert of Gembloux remarked that the Bulgarian state was established in 680. This was the first state that the empire recognised in the Balkans and the first time it legally surrendered claims to part of its Balkan dominions.