In the meantime the Bulgarians continued to strike Thrace, but in the autumn of 812 they were offered peace. The Bulgarian delegation was led by Dobromir, but the Byzantine Emperor refused to conclude peace due to "his foul advisors' suggestions" as Theophanes says. However the real reason was most probably Item 3 in the Byzantine–Bulgarian Treaty of 716 which stated that "The refugees deserters from both sides shall be mutually surrendered, if they are plotting against the authorities." That item was important for the Byzantines during the 8th century because the authority of their Emperors was weakened but, after the crisis in Bulgaria in the mid 8th century, it became inconvenient for them. In response to the refusal, the Bulgarians besieged Mesembria (Nessebar). They had excellent siege machines built by an Arab emigrant and soon captured the town where they found 36 copper siphons used to throw the famous Greek fire and a large quantity of gold and silver.