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. The main obstacle was the strong Byzantine-held fortress of Serdica (Sofia).
In the beginning of 809 Krum besieged the city but he could not break the resistance of the garrison for several weeks. In the end he promised to give safe conduct for the Byzantines on condition they yield the fortress. They agreed and Krum entered Serdica before Easter but he killed the whole 6,000 garrison and some citizens nonetheless.
The successful siege was of great importance for Bulgaria, because Serdica was a major crossroad in the Balkans. It was used as a main base by the later rulers of the country to expand their borders and influence to the south and southwest.