theme of the Byzantine Empire

Map showing the extent of the Byzantine Empire in c. 600 and c. 900, including the themes for the latter date

The themes or thémata (Greek: θέματα, thémata, singular: θέμα, théma) were the main military and administrative divisions of the Byzantine Empire. They were established in the mid-7th century in the aftermath of the Slavic migrations to the Balkans and Muslim conquests of parts of Eastern Roman territory, and replaced the earlier ancient provincial system established by Diocletian and Constantine the Great. In their origin, the first themes were created from the areas of encampment of the field armies of the East Roman army, and their names corresponded to the military units that had existed in those areas. The theme system reached its apogee in the 9th and 10th centuries, as older themes were split up and the conquest of territory resulted in the creation of new ones. The original theme system underwent significant changes in the 11th and 12th centuries, but the term remained in use as a provincial and financial circumscription until the very end of the Empire.