This was mainly because, after significant victories over the Arabs, the Byzantine Emperor Constantine V began to fortify his border with Bulgaria.

Khan Kormisosh took those actions, and the construction of a new fortress along the border, as a breach of the Byzantine–Bulgarian Treaty of 716, signed by Tervel.

[3] The Emperor himself, leading the main force, advanced into Thrace, and was engaged by the Bulgarians at the border castle of Marcellae.

The details of the battle are unknown but it resulted in a victory for Constantine V. In order to stop the invasion, the Bulgarians sent hostages to Constantinople.

[4] However, three years later (759), Constantine invaded Bulgaria once more, but suffered a crushing defeat in the battle of the Rishki Pass.