On Themes

«The themes were divided up and created when the godless Agarenes (Arabs) slowly began to encroach on the Roman Empire and dismember it. And up to the time of Emperor Justinian and the reign of Maurice, everyone was under the same administration. This is proved by Belissarius himself, who was general and commander of the whole of the East. But when the Agarenes began to campaign against the Romans, destroying their countries and towns, in each case the kings were forced to divide up unified administrative power into smaller parcels.

[Constantine Porphyrogennitus, On Themes (10^th century), A. Pertusi 1952, p. 60 - in Greek]