 The Byzantines hoped to take advantage of Bulgaria's instability at the time, caused by the presence of the underage Malamir on the throne, and broke the long-lasting peace established with the Byzantine–Bulgarian Treaty of 815, which they had initially reaffirmed upon Malamir's accession.In 836, Isbul was in charge of the Bulgarian forces which repulsed the Byzantine invasion and proceeded to raid into Byzantine territory.As part of this campaign, Isbul and Malamir captured the Thracian fortresses of Probaton (near Adrianople, modern Edirne) and Bourdizon (modern Babaeski).