At the conclusion of The Complete Roman Army, Adrian Goldsworthy provides a very brief summary of the role the army at the end of the western Roman Empire with a contrast to how the army helped sustain the eastern Roman, or Byzantine, Empire for another thousand years.
The Roman Empire in the west was gone by the fifth century while the Byzantine Empire lasted another millennium. By the way that provides a link between discussion on my blog of the Roman Empire and the Viking era.
Mr. Goldsworthy says at the end in the west its military was still stronger than its opponents. In spite of that strength, a combination of factors contributed to the end:
Yet weakening central authority, social and economic problems, and most of all the continuing grind of civil wars eroded the political capacity to maintain the Army at this level of effectiveness.
All those factors combined to wear down the financial ability and political will to maintain the army. After all:
A permanent, well-equipped, organized and disciplined professional army was a very expensive institution to support, but also a dangerous one, for no Emperor could ever be fully secure when there was the slightest chance of the army backing a usurper.
It was expensive to maintain a strong army. At the same time it was very dangerous. From other reading I’ve done, by this time troops were as loyal to their Legion commander as they were to the central authority. If you lost the support the commander there could all of a sudden be 6,000 well-trained and heavily armed warriors show up in Rome ready to take you on. If several commanders got together, look out.
In contrast, the Byzantine Emperor maintained this balancing act much more effectively:
The Byzantine emperors managed to maintain an army which combined a reasonable level of efficiency with a fair record of loyalty, encouraging long periods of political stability which in turn brought the prosperity which permitted them to afford this force.
This was astoundingly expensive, as I will at some point explain in terms of the high pay provided to Scandinavian soldiers in the Varangian guard unit.
It is an amazing simultaneous equation. An Emperor at the time had to spend a fortune to maintain the army, expanding that fortune even more to maintain loyalty, and wisely deploy the forces. All that was necessary to have the political and economic stability to provide economic development and prosperity which made it possible to in turn afford the Army. But to develop and maintain economic prosperity you had to have a powerful and stable army on a long-term basis.
Book concludes by saying that the emperors in the east were able to keep that simultaneous equation functioning for a millennium longer than the emperors in the west.