 The themes were not simply garrison troops, however, but mobile field forces capable of supporting neighboring themes in defensive operations, or joining together to form the backbone of an imperial expeditionary force for offensive campaigns.It was under this new system that the Byzantine army is generally considered to have come into its own, distinct from its late Roman precursor.The Thematic system proved to be both highly resilient and flexible, serving the empire well from the mid-7th through the late 11th centuries.