It’s seems that everything that has happened in battles over a thousand years ago for example Battle Of Cannae is so well documented. How did we know that a specific cavalry charge happened or a flanking manoeuvre ? Was there some sort of battle scribe that kept documentation on what happened ?
Cannae
Funny you should ask this question for this battle, as u/Tiako has examined how we know how Cannae went. While the response covers the Roman period, it's applicable to pretty much any period; the real challenge is in how to reconcile different accounts of the same battle. (Leuktra, ho!)
And of course, if anyone else would like to contribute further, please don't hesitate to write up! More posts are always, always welcome.