Krum took the initiative and in 812 moved the war towards Thrace, capturing the key Black Sea port of Messembria and defeating the Byzantines once more at Versinikia in 813 before proposing a generous peace settlement. However, during the negotiations the Byzantines attempted to assassinate Krum. In response, the Bulgarians pillaged Eastern Thrace and seized the important city of Adrianople, resettling its 10,000 inhabitants in "Bulgaria across the Danube". 

Infuriated by the treachery of the Byzantines, Krum ordered all churches, monasteries, and palaces outside Constantinople to be destroyed, the captured Byzantines were slain and the riches from the palaces were sent to Bulgaria on carts. After that all enemy fortresses in the surroundings of Constantinople and Marmara Sea were seized and razed to the ground. The castles and settlements in the hinterland of Eastern Thrace were looted and the whole region devastated. Then Krum returned to Adrianople and strengthened the besieging forces. With the help of mangonels and battering rams he forced the city to surrender. The Bulgarians captured 10,000 people who were resettled in Bulgaria across the Danube. A further 50,000 from other settlements in Thrace were deported there. During the winter Krum returned to Bulgaria and launched serious preparation for the final assault on Constantinople. The siege machines had to be transported to Constantinople by 5,000 iron-covered carts hauled by 10,000 oxen. However, he died during the height of the preparations on 13 April 814.