Just finished the book by that title, by Adrian Goldsworthy. Read Gibbon many years ago, and still have my copy. This isn’t Gibbon in it’s sweep, but it is a fresh look at a question for the ages. Why did the most technologically advanced nation on Earth, a nation that left traces of its industrial prowess in the polar ice thousands of miles away, lose so much technology and industry so quickly? Why did the strongest military power of its age, literally unmatched by any combination of its neighbors, eventually fall to relatively small bands of barbarians?
Goldsworthy’s primary thesis is that the political climate post-Marcus Aurelius, where civil war became the primary means of competing for and winning imperial power, resulted in the dissipation of Roman power.
Emperors could not trust in civil society to keep their throne safe. Their rivals with the most potential power, the Senatorial class, were gradually shunted aside from loci of power such as military and provincial commands in favor of lower-ranking men who were, in theory, more beholden to the Emperor. However, the lack of clear lines of Imperial succession led to more frequent civil wars and other contests for power such as coups. The more disordered the political landscape, the greater dispersion of power in order to reduce the ability of any one particular provincial leader to challenge the current Emperor. The greater the dispersion of power, the larger the bureaucracy and the more room for graft and featherbedding as more and more layers came between the Emperor and the power of the empire.
Additionally, the growth of bureaucracy led to an alternative route to power with less personal risk than the traditional route of a military career. At the end of a civil war, the military leaders on the losing sides usually were killed, but the bureaucrats associated with the same side may have had a greater chance of survival.
The constant loss of trained military personnel tied to the nearly continual state of civil war in the third century AD would also lead to a gradual decrease of effectiveness, blunting the gladius of Roman military power. As Roman military power declined, the need to recruit barbarians to “do jobs Romans wouldn’t do anymore” grew. The need for barbarian troops led to the incursion of additional barbarians. Eventually, the hollowed-out husk of the Roman military could not adequately control the borders and the Western Empire fell in a series of stepwise conquests by various barbarian tribes in the 5th century AD.
As for the Roman economy, again, the drain of constant civil war gradually eroded trade and drove movement of people from the less secure countryside into more secure towns. The money needed to pay for the warring armies led to currency debasement and inflation. All of these factors throttled trade, first impacting luxury items and specialty trade chains, then driving down the trade in staple items that, in the long run, held the empire together. The loss of Roman Africa to the Vandals in the 420s was the final nail in the coffin for the Western Roman Empire, while the loss of Egypt to the Arabs in the 640s had a similar, if not as extreme, effect on the Eastern Roman Empire.
Summing up, it appears to me that, in the long run, Romans were given multiple incentives not to learn how to fight, leading to a gradual loss of the skills and drives that built the Roman Empire.