The Eastern Roman Empire gave up a lot of its power when they decentralized the military in favor of the "theme" system. Ironically, this reform was undertaken

The Eastern Roman Empire gave up a lot of its power when they decentralized the military in favor of the "theme" system. Ironically, this reform was undertaken in order to more effectively combat the constant Islamic invasions that were taking place at the time (More at: http://en.wikipedia.org/wiki/Theme_system http://en.wikipedia.org/wiki/Theme_system).