The Byzantine Empire’s administrative and military districts were called themes. A theme was both a territorial division and a military command, reflecting the empire’s need to defend its remaining lands more flexibly during a period of repeated invasion.
The system developed mainly during the mid-7th century, after Byzantine defeats and territorial losses to Arab armies and the settlement of Slavic groups in the Balkans. Field armies were assigned to regions in Anatolia, and the names of some early themes originally referred to those armies rather than to clearly defined provinces. Over time, the term came to mean the district where an army was based.
Each theme was generally governed by a strategos, who combined important civil and military powers. This arrangement allowed taxation, local administration, recruitment, and defence to be coordinated more closely. The system reached its height in the 9th and 10th centuries, when themes formed the backbone of Byzantine provincial government and military organisation.
Theme is not simply interchangeable with province. Provinces were older Roman administrative units, while an exarchate was a different type of territory governed by an exarch. Themes also changed over time, and by the late 11th and 12th centuries their original military-administrative role had weakened.