 During Justinian 's reign , the Balkans suffered from several incursions by the Turkic and Slavic peoples who lived north of the Danube . Here , Justinian resorted mainly to a combination of diplomacy and a system of defensive works . In 559 a particularly dangerous invasion of <unk> and Kutrigurs under their khan <unk> threatened Constantinople , but they were repulsed by the aged general Belisarius . 
