Most ancient history buffs have at least a passing familiarity with the major periods of Roman history. The founding myth of the Roman Republic is that is was originally ruled by kings until the kings became too tyrannical, at which point they were deposed and the city became a republic, with elected officials serving fixed terms in office. Hundreds of years later Rome was transformed into an empire, with hereditary emperors ruling for life, driven by Julius Caesar, his great nephew Augustus, and their allies and opponents in the civil wars of the first century BC. Broadly speaking, we divide Roman history into three major parts: the Kingdom, the Republic, and the Empire (forget about the stuff after the East-West split for now; it's complicated and not relevant to today's topic). I want to go over the typical layman's picture of how the transition from Republic to Empire happened (or at least the picture of it I had in my head) and show how my view has changed since starting to read Tacitus' Annals.

The Republic

In Republican Rome, laws were made by the Senate. Various ranks of magistrates—censor, consul, proconsul, praetor, tribune, etc.—performed various executive functions of government. The consuls were the heads of state, something like US presidents, except that they served one-year terms and there were always two consuls each year. Occasionally a consul would appoint a dictator to lead the state in times of emergency, which granted the dictator basically absolute power in order to do what was necessary to save the state (amazingly, many Roman dictators willingly resigned their power after the emergency for which they were appointed was past).

Transition to Empire

Very briefly, the typical belief about the transition seems to be something like the following. A lot of upheaval and civil wars happened in the late second and early first century BC. Eventually Julius Caesar crossed the Rubicon in 49 BC and took control of Rome, then defeated Pompey's forces in many battles over the next few years, consolidating his power and becoming an emperor in everything but name, but his ambitions were cut short when he was assassinated on the Ides of March, 44 BC. Chaos prevailed and the civil wars continued until Caesar's great nephew Gaius Octavian assumed the office of princeps in 27 BC, becoming known as Caesar Augustus, the first Emperor of Rome. After that, Rome was an empire ruled by emperors. Simple, with a fairly clear dividing line between Republic and Empire.

Tacitus

Tacitus' Annals covers the period of Roman history starting with the very end of the reign of Augustus in AD 14; The first six books focus primarily on Augustus' successor Tiberius. I've only read through book 3 as of writing this, so this isn't meant to be an in-depth look at the Annals, but there's something interesting I've noticed thus far in my reading: the government institutions that existed in the Republic—the Senate and all of the magistracy ranks—still exist in the Empire under Tiberius. Tacitus begins his account of each year by telling us who the consuls for that year were—a fairly typical way Roman historians would delineate years. He recounts many of the goings-on in the Senate—trials, policy changes, and the like. It all seems like business as usual for the Romans in much the same way they did things pre-Julius Caesar, with the major exception that there is now a new office called princeps, occupied currently by Tiberius, which is a lifetime position holding absolute power and commanding the deference of all other institutions.

So all this got me thinking. The vestiges of the Roman Republic are clearly still there for at least a half century after the official beginning of the Empire in 27 BC (and probably longer; I don't actually know how long this lasts because I haven't read that far yet). How different would things have actually looked to the people living through all this? If you were an average Roman living in the few years before Caesar crossed the Rubicon, you would have seen the government working a certain way. If you then time traveled to let's say AD 17, a few years into the reign of Tiberius, what would you have seen? The Senate still convenes and votes; the consuls are still elected; proconsuls still govern provinces; the government works in much the same way it did before, only now there's a princeps. The difference between Republic and Empire seems obvious to us, but how obvious would it have been to people back then? I'm not sure, and it's unclear from my reading of Tacitus so far.

Possibly a more interesting question is, why do the emperors in their absolute power keep the government institutions largely unchanged (at least for now)? I can think of a few possible reasons:

1. They need a mechanism or apparatus by which to run the Empire; this one is already here, so why reinvent the wheel?
2. Power can be a fickle thing; they need support from other powerful people, and especially from the military, to maintain their power and not get deposed or assassinated. Overhauling the whole thing all at once could ruin their support and lead to their downfall.
3. In a similar vein to number 2, maintaining a façade of normalcy keeps the regular people content and promotes stability. If the people don't notice they are now ruled by an autocrat, maybe they'll be less likely to try to upset the status quo.

The vestiges of the Republic are still there, but there has clearly been a major change that will have ramifications for hundreds of years to come. The key here is that major changes in governmental function and practice can happen while the structure of the government itself officially remains nearly the same.

Now here's the real point. The United States Constitution has gone relatively unchanged since it was ratified in 1787 (except for the amendments). Possibly the biggest official structural change was the 17th Amendment in 1913, which established direct popular election of US Senators (they had formerly been elected by their state legislatures). On paper, the US government works largely the same as it did in 1787. However, we know from the Roman example that governments can undergo major changes while keeping the vestiges of their old versions intact. How many major functional or practical changes has the US government gone through since the Constitution was ratified? I leave this question as an exercise for the reader.