CEVAP:

The theme system was created to defend each of the provinces of the sprawling Eastern Roman and then, the Byzantine Empire. These themes were established in the middle of the seventh century following the Slavic invasion of the Balkans. They were also used after conquest of parts of the Byzantine territory by Muslims. The themes were created from the areas of encampment of the armies of the Roman Empire, and they were given the names of the Roman military forces that had previously occupied regions.