"They Shall Not Grow Old" is probably the closest popular documentary I can think of to your ideal (that focuses on humans at least) -- and it still implicitly needs to weigh in on some narratives, for instance the evolving thinking on "did the generals undervalue their soldiers' lives or were they learning as fast as they could?".

If you really care about what happened at X, you have to read/watch/listen to multiple sources and synthesize. No one will do that for you without injecting their own conclusion.

fwiw, watching 2 documentaries and reading a HN thread is already more research than I ever expected to do on a failed festival.