What was the name of the Byzantine system of military provinces?

The Byzantine system of military provinces was called the theme system. Its provinces, known in Greek as themata, combined military organization with regional administration and became a defining institution of the middle Byzantine Empire.

The system developed during the seventh century, after Byzantine losses to Arab armies and Slavic migrations drastically reduced imperial territory and manpower. Field armies were settled in regions—especially Anatolia—that became associated with their military commands. A theme was governed by a strategos, who generally held both civil and military authority.

The system helped the empire respond quickly to raids because soldiers and commanders were based near threatened frontiers. Soldiers, often called stratiotai, received access to land and were expected to maintain military readiness. The earliest major themes included the Anatolikon, Armeniakon, Opsikion, and Thrakesion.

A common mix-up is treating themes as purely military units. The word originally referred closely to army commands, but it increasingly described territorial provinces as well. Themes remained important through the ninth and tenth centuries before later emperors divided and reorganized them.