I'm often impressed by the exacting detail that seems to be available for the people and events involved, like Caesar, Pompey, Cicero, Augustus etc. In fact I'm impressed to the point that it seems a bit hard to believe, when it comes to ancient history I'm much more used to a huge degree of muddiness to our understanding of events and the personalities involved, like the accounts of Alexander the Great seem to be shrouded in mystery and centuries of propaganda that came afterward, and thinking about Rome itself for the periods that followed the End of the Republic there seem to be some quite noticeable holes, for example, even though their reigns seem to have been peaceful, prosperous and long it seems as though we have a real dearth of information when it comes to Trajan, or Antoninus Pius. I can at least understand better why, say, the crisis of the third century would be relatively poorly documented, but the late republic seems to be incredibly well-resolved in our understanding considering how long ago it was. I also get the impression that documentation of the period, to a certain extent, cuts through the winners attempts to control the narrative, I imagine people might scoff at me saying so considering Caesar's self documentation in his Gallic War, but for example we are quite well aware of the negative aspects of Caesar and Augustus and who their enemies were and what they were often saying about them. In addition its kind of amazing to me that we have such a direct line to the individual of Caesar through a work like that, again that seems radically different compared to other Titans of the ancient world like Cyrus or Alexander.
So I guess the gist of the question is just how do we end up in this position where we seem to know so damn much about the late Republic, in comparison to other times in Roman history or other parts of the contemporary world? Were the events leading to the transition to empire just so damn important that people made sure they were documented with more clarity than they might otherwise? Were they so important that records were continuously copied and studied to keep this history clear and accurate in peoples minds for centuries afterward?
To some extent I could answer this question by invoking a single name: Marcus Tullius Cicero. The years of Cicero's legal and political activity coincide exactly with the final decades of the Roman republic, from the fallout after Sulla's dictatorship (Cicero's first recorded speech is the Pro Quinctio, dating to 81 BC) to the year after the assassination of Julius Caesar and the rise of the Second Triumvirate (the last datable letter we have is from July 28th 43 BC). The volume of surviving work by Cicero is truly impressive: over 900 letters (800 by Cicero himself and 100 written by others to him) 52 speeches (mostly judicial, but many political ones too) and around 20 works on philosophy, politics, religion and oratory. Even all of this is only a fragment of what he actually wrote in his incredibly prolific lifetime.
What's more, Cicero was no supporting act or sideline observer, but was a crucial figure throughout the period with a position right at the heart of the action. This gives us a level of insight into the political, religious and social events of the late republic that it totally unparalleled for other periods of Roman history, perhaps for Antiquity as a whole. At times from his letters, we have an almost daily update of events. When Cicero was wracked by uncertainty over which side to join at the outset of the civil war between Caesar and Pompey, he was in constant contact with his best friend Atticus, updating him on the unfolding events and asking for his advice. Between September 44 and April 43, Cicero made 14 speeches denouncing Mark Antony (collectively known as the Philippics) again commenting on the rapidly changing events and giving us a level of detail we would not otherwise have.
Of course, Cicero is only one man, and writes from a very particular perspective. We certainly don’t have a dispassionate or unbiased account from his work (or even an accurate one at times), so must treat it with caution. Thankfully there are other writers we can consult to balance out the picture. The work of other contemporaries also survives, although not in the same quantity. Sallust’s histories of the Jugurthine War and the Catilinarian Conspiracy have survived complete and we have fragments of his other work. We also have fragments of Cornelius Nepos’ biographies, including one of Cicero’s friend Atticus. You mentioned Caesar in your question – alongside the Gallic Wars we also have his account of the Civil War against Pompey, as well as works on the Alexandrian, African and Spanish Wars that were sometimes attributed to him but are probably by others, maybe on his staff. Naturally these are incredibly partial, but when compared to other periods of Roman history it’s fascinating to have *anything* written by the men at the top of the political system. I mean, what do we have from later emperors? Augustus’ *Res Gestae*, a few letters of Trajan to Pliny the Younger and Marcus Aurelius’ boring philosophy? Please.
After these, we rely on Greek historians and biographers of the Imperial period. Appian’s *Civil Wars* cover most of the period from the Gracchi up to the wars of the Second Triumvirate and Cassius Dio’s massive *Roman History* is relatively intact from 69 BC onwards. Plutarch’s biographies of Caesar, Cicero, Crassus, Cato the Younger, Pompey, Antony, Brutus and others are immensely valuable for piecing together the narrative of the period.
As to why all this stuff survives, it’s a very good question. The fall of the republic attracted attention in antiquity because it was recognised as an important period in Rome’s history. Later writers were attracted by the huge personalities of the individuals and the dramatic events that unfolded, as well as trying to explain the change from republic to empire. I expect that these are the reasons behind the survival of many of these works over the two millennia since the first century BC, although the quality of the work plays a part. Cicero has always been considered an authority on oratory, as well as a philosopher who is worth reading.
I should say that it’s not a perfect picture, and as always, we have lost far more than survives. Cicero wrote many many more letters that were not preserved, either by the original editor (probably his freedman Tiro) or by later copyists. Livy’s history is almost entirely lost after 167 BC, even though the original work went up to the death of Cicero, and the work of other contemporaries like Posidonius are also gone. Dio's work is incredibly fragmentary for most of the republic after Rome's early history. The problem is worse before Cicero’s letters and speeches get going, and particularly serious for the 70s BC where we have almost nothing. The 80-odd years from around the destruction of Carthage (146 BC) to the start of the surviving letters of Cicero (in the 60s BC) is fairly thinly covered, at least in terms of the year-to-year political developments in the city. We've got biographies of the Big Men (the Gracchi, Marius, Sulla etc), and the military situation is well-served, but little else. Away from literary sources, we’re surprisingly poorly served by both epigraphy and archaeology for the late republic, meaning we’re missing vital checks on historians and biographers.