system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How do we know how certain battles took place?

I just watched the newest HistoryMarche video and it got me thinking. How do we know the details of a battle so thoroughly that we can make a video and present it? Like what flank collapsed, what flank held on.

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I understand primary sources, secondary sources etc. but how would someone know. Was there a guy just interviewing the survivors after the battle, sitting on the hill watching and noting the battle?
gpt
Depends on the battle.  For more recent conflicts like the colonial wars of the 18th century and later there is often meticulous note-keeping by commanders on both sides.

During the imperial age nations understood the immense value of detailed understanding of your army, your opponents army, and the relative strengths and weaknesses of each.  Military leadership tasked officers with this specifically, and commanders filed post-combat reports detailing their actions and thoughts.

As a result, we have *extremely* detailed records of battles in conflicts like the US Civil War and WWI, right down to the exact number of men and what time of day things occurred.

Once you get into pre-industrial conflicts the record quickly gets a lot spottier.  Records are sparse, lost, incomplete, and/or heavily embellished.  Many medieval and classical battles are known only from a single secondary source and some archaeological evidence, and the numbers and details some guy wrote about 45 years after the fact are deeply suspect.