There is a little book by Montesquieu on the Roman Empire, old and byzantine, which I happened to read just now (original title: Considerations sur les causes de la grandeur des Romains, et de leur decadence). It is an interesting little book, although the conclusions are not very sound. Montesquieu calls the Byzantine period of the Roman Empire, Greek, and prefers to speak about “the Greek empire”, without denying of course the continuity with the old empire. According to Montesquieu Byzantium fell mainly for these reasons:
First, because Byzantium was too religious and too sentimental with religion, contrary to the warlike religious fanaticism of its enemies (e.g. Montesquieu quotes an incident when emperor Mauricius was crying before the battle, thinking how many soldiers were going to be killed).
The second reason is a variation of the first. Because of Byzantium’s passion with faith, political unity had a secondary importance, and in the emperors’ attempt to keep Christianity many non-Christian people or heretics worked for the breaking of the empire.
The third reason is that the people of Byzantium were not obedient to the authorities, frequently ignoring the will of the emperors and doing what they liked. The fact, also, that in Byzantium anyone could become emperor (e.g. Justinian came from an insignificant family of peasants), tended to cultivate to the people rather familiarity with the emperors than fear of them, which in turn favoured anarchy.
The fourth reason is a variation of the first and second reasons, it is monasticism and its influence upon society. Montesquieu refers the example of emperor Andronicus Palaeologus, who was afraid that God may not like him because he was occupied more with the affairs of the state than with spiritual affairs. Montesquieu also refers to the final days, when people were more interested, even then, in theological discussions on the occasion of the uniate council of Florence, than in the approaching hordes of the Turks.