It may surprise you to note that the Fall of the West, as viewed by the Byzantines, appears to be the basis for much of our modern historical interpretations on the era. It is known to us because many Byzantine authors took the time to write small asides telling the reader why the Roman Empire no longer controls certain lands. Works such as Theophanes' Chronicle highlight much of the History of the Roman Empire, and so the Byzantine people were generally very aware of the historical reasons for why their Western lands had been lost.

Most of the authors explain that "the neglect of the ancients" was the cause of the Fall of the West, which somewhat echoes the comments made by Vegetius in his work De Re Militari, written in the 5th Century. This "neglect" takes many forms, from neglect of the duties of the Emperor, to neglect of the furthering of social values, to neglect of the training of the troops and the military. It is interesting too, because this "neglect" is virtually identical to the commentary made by the authors on more contemporaneous topics, which perhaps reveals that Byzantine authors recognized the repeating of history in various events. To begin to understand some of the theories, let's begin with Vegetius' comments on the matter:

"The name of the legion remains indeed to this day in our armies, but its strength and substance are gone, since by the neglect of our predecessors, honors and preferments, which were formerly the recompenses of merit and long services, were to be attained only by interest and favor. Care is no longer taken to replace the soldiers, who after serving their full time, have received their discharges. The vacancies continually happening by sickness, discharges, desertion and various other casualties, if not supplied every year or even every month, must in time disable the most numerous army. Another cause of the weakness of our legions is that in them the soldiers find the duty hard, the arms heavy, the rewards distant and the discipline severe. To avoid these inconveniences, the young men enlist in the auxiliaries, where the service is less laborious and they have reason to expect more speedy recompenses."

You will find this same viewpoint to be echoed in many of the maxims highlighted in Leo VI's Taktika. In fact, one could argue that the Strategikon (of Maurice) and the Taktika (of Leo VI) were written in part to prevent the decay of the Byzantine armies in the same manner as that of their predecessors.

And, you will also see lines such as this one, from Constantine VII's De Administrando Imperio: "And since, as we said above, owing to sloth and inexperience of those in power, things had gone sour for the Romans" This highlights that the Byzantines attributed the poor decision making of leaders during critical events to the decline of the Empire throughout its history. Additionally, Constantine goes through a lengthy explanation of the overrunning of the Western Empire by the Goths and the Vandals in the aforementioned work, which shows that he was well aware of the events and how they took place. Again, the Byzantines had access to the best available sources on the history of their Empire, and so, it is natural to assume that they mulled over the topic for a very long time and came up with these many various theories on their own - just as we do today.