 This new challenge, which seriously threatened the empire's survival, compelled Heraclius and his immediate successors, in the mid-7th century, to undertake a major reform of the Byzantine military system to provide for a more cost effective local defense of its Anatolian heartland.The result was the theme system, which served as both administrative and military divisions, each under the command of a military governor or strategos.The theme was a division-sized unit of around 9,600, stationed in the theme (administrative district) in which it was raised and named for.