A strange historiographical situation
By the early second century CE, the Roman Republic had been gone, in any operating sense, for more than a hundred and fifty years. Augustus had been dead for nearly a century. The senators and equestrians who made up the literate class of the high empire had been born under the principate, served it, prospered inside it. The institutional life of the Republic was, for them, a memory of a regime they had never seen working.
And yet the historians of that generation — Tacitus most prominently, but also Suetonius, Plutarch (writing in Greek but for Roman readers), the imperial-era epitomators of Livy, and the long line that runs into Cassius Dio in the third century — kept the Republic as their principal subject matter. The Annales opens by sketching the Republican past whose end it then describes. The Lives of the Twelve Caesars begins with Julius and the fall of the Republic. Plutarch's Roman Lives are dominated by Republican figures — the elder Cato, the Gracchi, Marius, Sulla, Pompey, Cato the Younger, Brutus, Antony — read for their moral and political character by a Greek living under Trajan.
The historiographical situation is unusual. A regime that had replaced the Republic was actively producing the most considered literary monument of the Republic anywhere in the surviving corpus. What was the work doing?
What it was not doing
It was not, on its surface, a Republican opposition movement. The imperial regime, particularly under the Nerva-Antonines, did not treat the historical reading of the Republic as politically dangerous; the work was published openly; some of it was performed in formal recitations; the authors held senior offices. Tacitus was consul; Suetonius was ab epistulis in the palace; Pliny the Younger, who admired both men, was a senior governor. The Republic was a permissible subject of historical study under the high empire in a way it would not have been under, say, Domitian.
It was not antiquarianism either. The authors are not principally interested in the Republic as ancient curiosity. The narrative pace is fastest, the analytical attention sharpest, the prose densest, on the political events of the late Republic — the moments at which the older constitutional vocabulary was being broken, repaired, broken again, abandoned. The writers are working over the same political question their contemporary regime has answered in its own way, and they are working over it deliberately.
It was not nostalgia in the sentimental modern sense. None of these writers proposes that the Republic be restored. None of them treats the Republic as a lost golden age. Tacitus is explicit at several points that the late Republic had been failing for at least a generation before Caesar; Plutarch's Pompey and Caesar and Cato are not innocents; Suetonius's Julius is recognisably the man Sallust had treated half a century earlier. The Republic the imperial historians are remembering is the Republic the Republic itself had not been able to keep going.
What it was doing
The work was doing something more interesting. It was keeping in circulation a political vocabulary — the constitutional terms, the moral categories, the institutional habits — without which the present regime could not be assessed. By writing the Republic's history in the patient and analytical register of high-empire prose, the historians made available to imperial citizens a set of concepts the imperial regime would otherwise have allowed to atrophy.
Three concepts in particular survive through the historiography:
Libertas — the older Roman idea of the citizen's standing relation to the polity. Under the empire it had taken on a different content (a citizen's protection against arbitrary imperial action; the absence of obvious humiliation; the dignity Tacitus catalogues losing through senatorial flattery); but the word and its constitutional charge survived in the historical writing because the Republic's historians had to use it to describe what they were describing.
Res publica — the Republic, the public thing, the commonwealth. The phrase remained in use officially under the empire (it was politically advantageous to claim that the res publica had been restored by Augustus) but the historical writing kept alive the older content the phrase had carried: the polity in which authority is shared and the citizens are the ground of legitimacy. The word running through Tacitus could not be reduced to its imperial honorific.
Mos maiorum — ancestral custom, the unwritten constitution of inherited norms. The Roman tradition had treated mos maiorum as constitutionally binding; the empire had no working substitute for it; the historical writing preserved the idea that custom can hold a polity together where written law cannot, and that custom is itself a political achievement that requires sustained civic effort to keep alive.
Each of these concepts continued to function in the imperial period because the historiography continued to use them in their older weight. The work was a holding action on the political vocabulary itself.
What the European tradition kept
The European reception of the imperial historiography of the Republic is one of the longer chains in Western political thought. The medieval and early-modern Christian readers received the Republican material through Tacitus, Suetonius and Plutarch; the Renaissance republicans (Machiavelli, the writers around the Florentine republic, the long Italian humanist tradition) worked through the same body of writing. The English commonwealth writers read Tacitus. The American founders read all three. What they received was not the Republic itself — that had been gone for two thousand years — but the political vocabulary the imperial historians had kept alive because they refused to let the words die with the regime that had originally used them.
Why the platform reads them this way
The platform reads Tacitus, Suetonius and Plutarch as the imperial historians of the Republic — that is, as the writers who took working political memory as part of their work even when the regime they wrote under was content to let it atrophy. The practice is the useful precedent. Whether the political vocabulary of self-government can be kept alive inside a regime that does not require it is a question every later European tradition has had to address in its own form. The high-imperial historiography is the most considered ancient case for the answer being yes, but only if it is done deliberately and across generations. The platform takes the practice seriously because the question has not retired.