Justinian's Flea by William Rosen describes the fall of the Roman Empire from the division of the Empire by Diocletian to the start of the rise of the Islamic powers. Rosen's book is one of many that has attempted to make sense of the time of transition between the end of the Roman Empire and what we call Medieval Europe. The Roman Empire was split up by Diocletian into east and west with Rome being the dividing line. It is from this starting point that Rosen starts to present his argument, from the split Rosen shows us the rise of the eastern roman empire and the rise of the city of Constantinople into a powerhouse in the eastern Mediterranean. At the same time Rosen describes to us the fall of the west and the rise of other powers in the western Mediterranean especially the Goths, Vandals and Huns. The rise of the East coincides with the fall of the West and the Byzantine Empire that would emerge from the city of Constantinople would become the heart of the remainder of the book. .
The rule of the Emperor Justinian is the central time frame in which the book is placed; this is when the Byzantine Empire was at its height. Rosen is clear in stating that the Byzantine Empire is not the Roman Empire but in fact is unique in its culture its laws and its architecture. The Byzantine Empire also was the important for its military victories over the Goths, and Huns. Belisarius was the leading general at the time when he and his army along with the financial backing of the Empire due to prosperous trade would reclaim much of the territory that was lost to the Goths and Vandals by the Western Roman Empire. The reclaiming of these territories would give the Byzantine Empire a vast geographical region to control with many others looking to take it down, however due to the effectiveness of the Byzantine Empire's armies and their masterful ability to control through diplomacy it was able to hold off invaders until the year 540.