By 1000, Basil had fought off his own nobility and defeated the Islamic threat from the east, and so led another invasion of Bulgaria. This time instead of marching into the middle of the country, he annexed it bit by bit. Eventually, after denying Bulgaria of about a third of its land, the Bulgarians risked everything in one battle in 1014.

The Battle of Kleidion took place in the valley between the mountains of Belasitsa and Ograzhden, near the modern Bulgarian village of Klyuch. The decisive encounter occurred on July 29 with an attack in the rear by a force under the Byzantine general Nikephoros Xiphias, who had infiltrated the Bulgarian positions. The Battle of Kleidion was a disaster for the Bulgarians and the Byzantine army captured 15,000 prisoners; 99 out of every 100 was blinded and the 100th was spared one eye to guide the rest back to their homes. The Bulgarians resisted until 1018 when they finally submitted to Basil II's rule.