After the destruction of the Avar Khaganate, Khan Krum turned his gaze to the southwest to liberate the Slavs who populated the valley of the Struma River and Macedonia.

In 807, the Byzantine emperor Nikephoros I marched against Bulgaria but was soon forced to return to Constantinople due to a mutiny of his troops at Adrianople.

[1] The following year, the Bulgarians, in response to an equestrian raid in the valley of the Struma River, defeated the Byzantines and captured their baggage.

The garrison held out for several weeks even as food supplies shrunk and the number of men declined due to desertions and the release of mercenaries.

In addition, Serdica was used as a primary military position by later rulers of Bulgaria to expand borders and spread their influence.