The longstanding absence of any effective leaders needed to strengthen both the economy and the military could be argued as an important factor that led to Rome's decline.
One of the reasons why Rome transitioned into an empire in the first place was because the leadership, (be it the consuls, Senators, tribunes, etc.) were unable or unwilling to mitigate the republic's persistent economic instability, which in turn undermined the country's political and military institutions. The latifundia system kept the small farmers and plebeians in debt to the patricians who owned most of Rome's land. If the lawmakers had accepted reforms, such as the proposal made by the Gracchi brothers to redistribute land, maybe that would have maintained the people's confidence in their government. But since those reforms were ignored, that inequality continued and grew so pervasive that it actually invited more ambitious men, like Sulla, to exploit the poor by offering them promises of obtaining land if they joined his own private army, thereby giving him the power to eventually seize Rome and wipe out dissidents. This, of course, would foreshadow Caesar's march on Rome.
Even when the Empire is in place, it is noticeable how the presence of competent as opposed to incompetent emperors impacts Rome's internal and external security. The "Five Good Emperors" (Nerva, Trajan, Hadrian, Antoninus Pius, and Marcus Aurelius) helped the land interests of the poor, provided state funds for their assistance and education, invested in public infrastructure projects (ie. aqueducts, bridges, roads, and harbor facilities) and strengthened the borders of the empire to the extent that Rome experienced almost a century of peace (the Pax Romana). After Marcus Aurelius dies, we see a series of emperors who are only concerned about maintaining military rule over the people. During and after the time of the Severan rulers, there was a decline in trade, labor shortages due to a plague, a decline in farm production and the currency was on the brink of collapse. This in turn would again undermine the strength of the government and military, for similar reasons that were encountered when Rome was a faltering republic - hiring soldiers would be a problem if they cannot be paid. And if the military is weak, there would be no means of suppressing civil wars, foreign invasions, and the trespassing of borders all of which was prevalent at the time.
While it's arguable that no single leader could have easily taken on such a formidable role during Rome's later stages, it should be recognized that Diocletian was able to resuscitate some stability into the Empire and Constantine's creation of the Eastern Roman Empire lasted for another millennia. In addition to strengthening the military, Diocletian brought some political stability with a power-sharing arrangement that divided the empire into four parts. Despite these accomplishments, the economic reforms of both men (wage and price controls, forcing workers to stay in their jobs) would not be enough to save the Empire.
But the fact that these leaders were able to make some significant headway at that point in Rome's decline suggests that it was possible for Rome to have been revived to an equal extent, if not saved entirely, had more leaders of that caliber arisen to power. Therefore, the chronic lack of such leadership, and the continued presence of weak rulers, is the most significant reason for its collapse.
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