The Crisis of the (mid) Third Century is a critical point in the History of Western Civilization. For those unfamiliar with the Crisis, it essentially was a fifty year long period from 235 AD, when the last Severan Emperor was assassinated by his own troops, to 285 AD, when Diocletian finally restored order and stability within the empire.
The Crisis of the Mid-Third Century set the tone of the beginning of the decline of the Roman Empire. Over two dozen Emperors occupied the Roman Imperial Throne in a span of fifty years, while in the previous reigns of the Five Good Emperors and the Severan Dynasty were not nearly as unstable. The Crisis also set the stage for economic chaos as the infrastructure of the Western Roman Empire completely broke down, with roads and aqueducts falling into disrepair. Localism and the warlordism that dominated the Dark Ages and the early Medieval Ages rose as a result.
However, to prevent the Crisis of the Mid-Third Century, we do have to time travel and either prevent Emperor Septimus Severus’ massive enlargement of the Roman military or prevent him from devaluing the currency and possibly allowing the government to fall into debt (bar it raising taxes). Emperor Severus, while a brilliant tactical and strategist, set the stage for the economic ruin of the empire when he devalued the currency by decreasing the silver purity in each Roman coin, or Denarri. A few decades later, inflation would begin to rocket
While a strong and just leader, Emperor Septimus Severus
The Crisis of the Mid-Third Century officially began with the Roman military’ assassination of Emperor Alexander Severus, a competent but war-weary emperor, which led to multiple military coups and assassinations that dominated the next half century.
Preventing the Crisis of the Third Century requires many aspects of history to change, but it could greatly lengthen the lifespan of the Roman Empire, especially its Western provinces. The transition from Classical to Medieval Europe might be delayed by decades, if not centuries, if the exact events of the Crisis never occur. While the constant civil wars were possible to recover from, the decentralization and localism wasn’t.
An iconic aspect of the crisis was when generals/governors in Gaul and the Levant tried to declare independence around the 260s to the mid 270s AD.
We can’t be sure how exactly the Roman Empire goes about after the crisis is averted. For all we know, Rome might develops the natural strength and flexibility after lasting for centuries longer in this timeline, that it essentially becomes the Western counterpart of China. With the lost provinces constantly reconquered by the remnants of the Imperium.
The most likely result is that when the barbarians try to flood the Roman Empire in the late 4th century due to the threat of the Hunnic Empire, the Romans might be more economically and politically stable to withstand their assaults like they had done against previous barbarians armies. They might lose some of their outer provinces, but personally I think Rome might be able to hope out a lot longer if they can have the strong infrastructure they had in the West before the Crisis and just hold Africa. I can’t be sure about this, but I believe the African provinces of the Western Roman Empire were the lifeline of the late Empire in the west. Emperor Majorian, the last good emperor of the Western Roman Empire, tried unsuccessfully to reconquer the African provinces after the Vandals overran them. As they were the breadbasket of the Roman Empire in the West, Africa would be essential to the survivor. Since the barbarians are probably bogged down in say Gaul or Britannia, the Romans will hold Africa longer and thus survive much longer.
Emperor Majorian attempted to destabilize the Roman Empire by reconquering lost provinces, but failed to retake the vital province of Africa.
See, the crisis set the beginning of the fall. How can we really envision the fall of Rome if we don’t have a beginning? Or worse come to worse, the Western Roman Empire falls in the late 6th or early 7th century. Rome may still fall, or it might not. What we can be sure is that the problems of the 5th century will probably fail to topple the Western Roman Empire due to the absence of a crisis on the scale of that which plagued of the third century.