Perhaps one mystery about the Roman Empire is not why it fell, but why it ever stood in the first place. In contrast to the Roman Republic, the empire contained within its very structure, or lack thereof, the seeds of its own destruction.
This inherent instability was crystallized in the question of succession. Because the empire was structured around the empty pretense of continuing the republican form and procedure, no clear procedure for, or line of, succession was codified.
The result was a built-in motive for assassinations, and an increased likelihood of power struggles, if not civil wars, between competing pretenders to the throne.
That there was never a smooth transition of power, and that an emperor ever died a natural death, is something of a marvel in these circumstances.
The system did apparently work to a limited extent for the first five emperors, whom historians treat together as the Julio-Claudian Dynasty. Of these first five, one was indisputably assassinated: Caligula. Nero committed suicide in order to avoid assassination. Of the remaining three - Octavian, Tiberius, and Claudius - the evidence is ambiguous as to whether their deaths were natural or contrived.
Despite the dubious causes of death, the mechanisms, if improvised, for the transitions of power functioned relatively smoothly, up until the death of Nero in 68 A.D.
After Nero’s death, Rome lived through “the year of four emperors,” as it is routinely called, and problem of succession emerged as one of the clear weaknesses of the imperial government.
This tumultuous pattern of succession would continue for many years, interrupted occasionally by bits of stability. Historian Ernest Gottleib Sihler describes the situation among the emperors of the early third century:
Caracalla, the cruel elder son of Septimius Severus, perished through Macrinus, commander of the Imperial Guard, in 217 A.D. In the very next year this short-lived Emperor was in turn slain while fleeing from the unspeakable Elagabalus, priest of the Sun and incarnation of every possible form of sexual depravity. This monster in turn was killed by his own praetorians after the world had endured him for four years, in 222 A.D. A nobler youth succeeded, known in history as Alexander Severus, but he, too, was done to death by his own troops, on the Rhine, in 235 A.D.
That the empire functioned for nearly five centuries is perhaps due to the efficiency of the civil service. The bureaucrats at the middle and lower levels kept the system running.
The instability in the succession process was matched by instability caused by powerful tribes who threatened the borders of the empire. The whole of these two problems was more their sum.
The attacking tribes created a need for a loyal and devoted military to defend the empire. But given the ambiguity about the emperor’s claim to sovereignty, such dedication was more difficult to find, instill, or call forth.
Ultimately, the emperors could rely only on the raw assertion of power to back up their claims to sovereignty. As long as they maintained the appearance of republican government - the senate met regularly throughout the centuries of the empire, even though its true power was microscopic - there could be no talk of dynastic succession or divine right. The emperors would also not tolerate the thought of being in any way confirmed or elected by the senate.
While the senate did formally declare some of the emperors to be emperor, this was again merely a formality for the sake of appearance.
Over the centuries of the empire, as Christianity went from being a ruthlessly persecuted underground movement to a legally accepted and acknowledged part of Roman society, the cultural impact of belief also impacted the power structure.
Historians diverge on the question of how the new faith affected Roman civilization: did it strengthen it or weaken it? Professor Sihler writes:
After this the emperors, one and all, were simply military pretenders, creatures of their own legions. None of them succeeded in establishing a dynasty. Persians, Goths, Sarmatians, Franks, Alemanni, began to overrun the frontier provinces of the Empire, the integrity of which was more and more threatened by its vastness. At the same time the inner unity and loyalty of the subjects were felt by the Roman officials to be gravely impaired by the aloofness of the religious sect ever growing at the cost of the idolatrous nations - felt perhaps by some statesmen of Rome to be a state within the state - the Christian church, an element of disintegration.
On the one hand, as Sihler notes, the followers of Jesus were perhaps at times less inclined to invest themselves fully in imperial power struggles and political machinations. On the other hand, the early Christians were less likely to seek power and initiate self-aggrandizement campaigns.
Finally, after the reign of Constantine, Christianity was given a recognized and legal status, and under the subsequent Christian emperors, Rome’s older polytheistic paganism was tolerated alongside Christianity. By ushering in an era of religious toleration, Roman unity may have been threatened by religious diversity, but energy and resources were not wasted in efforts to suppress or exterminate any one faith.
The net impact of Christianity on Rome, then, is ambiguous, or at least disputable. In any case, it was overshadowed by succession problems and by threats from external nations, among other challenges faced by the empire.