There's a huge difference, for an historian, between studying events which took place two hundred years ago, and those which transpired two thousand years ago. While there are piles and shelves and entire large libraries' worth of the original copies of documents written or dictated by the American Founding Fathers and the protagonists in the French Revolution and the Napoleonic era, taking quite a bit of the doubt out of our conceptions of what people like George Washington and Georges Danton really wished to communicate to their contemporaries, we don't have any original copies of the works of Julius Caesar or Livy or Ovid or Vergil, or any other of the Classical Latin authors. We don't have any copies made within three hundred years of the lives of the authors in the cases of Livy or Vergil. In the cases of Caesar and Ovid, enormously popular authors from their own time down to the present, no manuscripts have survived which are older than the 9th century, more than 900 years after the texts were first written, unless some older fragment on papyrus has been found recently in the Middle Eastern desert. Even the 9th century is very impressively old for a copy of an ancient text. For some of the most highly-prized writers of ancient Latin, we have to make do with manuscripts no older than the 15th century, or even more recent than that. But in such cases we still make do gratefully -- why? Because there are very many highly-prized ancient authors whose works we no longer have at all, about whom we know only because other writers have mentioned them. And in the cases of Caesar and Livy and Ovid and Vergil and many, many other ancient authors, we have mentions of other works they wrote, but no copies of those works. Rarely, a copy of one of those lost authors, or of a lost work by some author whom we already know, turns up. And that's also a big occasion, even if all we've found is a few words.
It'd be nice if we had all sorts of original or near-original copies of things written thousands of years ago. But we only have a few, and most of those are either inscriptions, words carved into stone like the Res Gestae Divi Augusti, a self-serving list, written by the Emperor Augustus, of his own achievements, carved into public monuments after his death,
or things like shopping lists and the personal letters of non-famous people on some of those above-mentioned scraps of papyrus, dug up at places like Oxyrhynchus in Egypt -- along with an occasional scrap of a very old manuscript of a text by someone like Homer or Vergil. When it comes to literary texts, Classical scholars attempt to reconstruct the earliest version of the text they can, as best they can. This process is known as scholarly editing, and rather than me clumsily attempting to explain to you step-by-step how it's done (I'm not ready to do it myself just yet), you'd be better off taking courses in Classical Studies, or referring to books like these, written by some of the world's leading Classical scholars:
The world's leading Classical scholars also edit texts for series like the Oxford Classical Texts (OCT) and the Teubner series,
Each volume in those two series begins with a preface in which the editor explains, often at great length and in great detail, which manuscripts and other sources of information he or she has used in the process of editing the text, and just exactly what he or she has done with those sources in order to come up with the present volume.
Of course, most of the prefaces in the OCT and Teubner series are written in Latin, which is 1) one more very good reason on the long list of reasons to learn Latin, and 2) perhaps the best single piece of evidence that, in the 21st century, Latin is still not dead.