The rule of Khan Pressian from 836 to 852 was marked by fierce battles, exhausting marches and swift military raids. No reliable records have been found regarding the internal policy of Omurtag's grandson. He succeeded his uncle Malamir to the throne after Malamir had ruled the country for just five years in peace with Byzantium. Pressian put an end to that peace. Byzantium was depleted by a bloody war with the Arabs. The Slavic tribes within the empire were rebelling and their chiefs were looking north to Bulgaria where they could find protection together with other Slavs. The Khan believed it was time to expand Bulgaria's territory to Macedonia in the southwest. His right-hand man was the old Kav-Khan Isbul who had been the indispensable aide of his grandfather Omurtag.

As soon an he ascended the throne in 837, Pressian sent his army under the command of Kav-Khan Isbul to the Aegean coast of Macedonia. There, they aided the Smolyans, whose revolt had shaken Byzantine rule in the Western Khodopes and Aegean Thrace. When the emperor sent an army against the rebels, his forces were met by the Kav-Khan-led Bulgarian army. Inscriptions on stone plates tell of fierce battles at Philippi and Siar, the elation of the victories reflected in the engraved words: "Pressian, through the will of God ruler of many Bulgarians...

Only the war against the Serbs ended in defeat. Waged from 839 to 842, this was the first war in the history of the two countries' relations. The defeat did not result in loss of territory.

Khan Pressian annexed vast territories in Central and Western Macedonia, including the towns Ohrid, Prilep and Bitola. Kossovo Pole also became part of Bulgaria. Thus in Khan Pressian's state, the Slavs from the "Bulgarian" group, inhabiting Moesia (Northern Bulgaria), Thrace and Macedonia achieved long-sought political unity.