At the end of the 8th century Bulgaria occupied the lands extending to both banks of the lower Danube, between the Carpathians and the Balkan Mountains. This a comparatively small territory had limited human, economic and military resources. At the same time Bulgaria was enclosed by powerful enemies: the Avar khanate in the north-west, the Khazars in the north-east and Byzantium in the south. These conditions did not promise particularly good future to the infant state.

At the beginning of the 9th century a new dynasty ascended the Bulgarian throne. The first ruler of this dynasty was khan Krum (803-814). Khan Krum laid the outset of a new foreign policy with a main purpose Bulgaria to become a state equal in territory, population and military strength to the European political giants in that time. The new Bulgarian ruler and his successors considered that the only way to realize this aim was to join forces with all the Slavs tribes on the Balkan Peninsula through including them into the borders of the Bulgarian state.

In 805 Charles the Great, the ruler of the Frankish empire, and khan Krum jointly destroyed the Avar khanate in Panonia. Bulgaria annexed its lands inhabited by Bulgarians and Slavs in Transylvania. After that khan Krum turned to the south-west and conquered Serdica (present-day Sofia) and part of Macedonia.

These actions provoked violent Byzantine resistances. The Roman emperor Nicephorus I at the head of a large army invaded the Eastern Bulgaria, reached to the capital Pliska, burned it down and killed off many old men, women and children. Khan Krum who was out of capital proposed to conclude a peace treaty. The Roman emperor denied. Then Bulgarian troops made an ambush at a narrow Balkan's defile. Here, on  July 26, 811 the Roman army was encircled and fully destroyed. The emperor himself was killed. Later khan Krum ordered to plate by silver Nicephorus' skull and celebrated together with his nobles the victories over Byzantium.

After this victory khan Krum turned his army to the south. The Roman fortresses in Thrace were captured one after another and in 813 the Bulgarian troops stood outside the walls of Constantinople. However, the Bulgarian ruler understood that it is necessary to have a large number of siege equipments in order to overcome the strong walls of the Roman capital. That is why he proposed peace. During the talks the Byzantines made treacherous attempt to kill him. The infuriated khan Krum devastated the entire South-Eastern Thrace.

Khan Krum began a strenuous preparation for definitive capturing Constantinople. He equipped enormous army and built numerous siege devices. Khan Krum created  unprecedented at this time for Europe heavy cavalry - 30000 horsemen and horses covered entirely by chain-armours.

At the height of these actions khan Krum suddenly died of a heart stroke in April 13, 814. But he left behind himself a powerful state, equivalent rival to the centuries-long Roman empire.