One mechanism for the transition from one state to the next is that the state's elites lose control of the succession process, and in this respect, history does often rhyme. Roman history provides good examples. The Roman Empire is usually divided into the Principate period lasting from the Battle of Actium in 31 BC, to the assassination of Severus Alexander by Maximinus Thrax in AD 235 that ushered in the Crisis of the Third Century. This was followed by the Dominate period, starting from Diocletian's administration in 284 to the fall of the Western Empire in 476.
Of these two transitions, the fall of the Western empire is better known. In AD 476, Odoacer interrupted the Western succession, or rather, he disintermediated it. As with most things in history, to contemporaries, things are either not actually so sudden, or they only seem sudden to isolated elites in denial due to normalcy bias. In this case, for a century at least, the Western emperor had largely been a figurehead for the Germanic magistri militum who held military power. When Odoacer killed the magister militum Orestes and deposed the last emperor (who was actually the magister's son), he was merely (finally) cutting out the middle man. At the time, contemporaries did not think much of the development, so naturally did it flow from the circumstances, and only later did historians begin to mark it as a major transition. But it was still the final removal of succession - even if the office was mostly ceremonial - from elite oversight, that is, from the generals who had been the de facto power in Rome during much of the Dominate.
Maximinus Thrax says Senators are cucks. "Just kill...I don't even wait." From imperiumromanum.pl
The Crisis of the Third Century is much more interesting. There were four dynasties during the Principate, but even when each of these ended, there were brief periods of at most a few years before stability returned. During the Principate, there was a tradition (if not what we would recognize as a formal institution) of future emperors being made the adopted sons of previous emperors, and then being chosen by the Senate. Earlier in the Principate dynasties, Senators frequently had military experience - they retained their connections with the legions and hence, the military's respect. But the trend during the Principate was toward less and less military experience among the elite families that produced Senators, and already by the Antonine dynasty, the military experience and connections of Senators had decreased dramatically. At its base, civilization is always backed by a threat of violence, but the more distant that threat is, the more pleasant our lives are and the more successful we could say the civilization is. By the time of the Severan dynasty, the emperors focused only on keeping the army happy (Alfoldy 1974), and when one side (the military) can use violence with no real threat of retaliation (from the Senate), there is clearly an unstable equilibrium. Thus, when Roman soliders assassinated the last Principate emperor Severus Alexander in 235, and then they decided Maximinus Thrax emperor by his legion rather than the Senate, we see another example of the final realization of elites losing the power to control succession. Maximinus never even came to Rome would be "emperor" (read: general who threatened to kill others and avoided being killed for long enough for contemporaries to bother writing down his name.) Maximinus was during his brief "reign" openly adversarial with the Senate, who rightly feared him when he did approach. This way of becoming emperor - a "barracks emperor" who had little interest in affairs in Rome or ability to govern the empire - continued until Diocletian's power-sharing arrangement in 284. The Crisis had begun.
The succession process in the United States would seem to bear little resemblance to that of the Romans. Indeed, our regular elections were set up partly with their example in mind as a warning. Though we've had leaders popular for their military record, it's not the elites’ changing relationship with the military that makes the analogy here. We choose our leaders through direct election by citizens, and since very early in the republic our choice has been constrained by two elite-controlled organizations called political parties presenting us with their candidates. It's difficult for most Americans to admit this, but this process (and until the early 2000s, the mass media oligopoly that supported it) is how the elites controlled succession - and it's even harder for us to admit that this may have been for the best. While the franchise has expanded over time, it's not obvious that each step in turn immediately changed the kinds of choices the parties were offering for national leader. Starting around 2010 the tone of politics and kind of candidates offered had, however, seemingly undergone a rapid shift, maybe not coincidentally now that everyone had a smartphone and social media.
This is how the elites have lost control of the succession process - in our case not by losing respect of and control over the military, but by a colluding media and party elite losing control of the information ecosystem and candidate selection process. The analogy here is that Trump is Maximinus Thrax, the first outsider who rose to power using a new process to circumvent the old elite-controlled pathway to leadership, with assassinations of character rather than physical bodies. This is why conventional candidates have been roadkill, like clueless, ossified Roman Senatosr during the Crisis, who might have continued maneuvering among their colleagues in Rome like their forebears did, not understanding how irrelevant such efforts had become - while generals far from Rome tried to get into position to assassinate the current one, wherever he might be encamped. If Gavin Newsom succeeds in turning Trump's social media appeal-to-the-masses approach against him, he will just be the next in a succession of social media barracks emperors - but the elites then will have lost control of the succession process, and the Crisis of the Twenty-First Century will continue.
Alföldy, G. "The Crisis of the Third Century as Seen by Contemporaries." Greek, Roman, and Byzantine Studies 15 (1974): 89–111.
