This paper discusses thirteenth and fourteenth-century Byzantine perceptions of civil wars, which were a common feature in the late Byzantine period. It investigates how the most important authors of the period understood and defined the idea of civil war. It explores the Byzantine understanding of the differences between military conflicts which were fought between subjects and employees of the emperor and wars the empire fought against its external enemies. In addition, it examines the views the imperial authorities and the authors of the period express about wars against enemies with whom the later Byzantines shared a common cultural, ethnic and religious background.