Lots of responses, but I'd mention a possible third option- a chapter epigraph. It's not easy to do well, but if you can condense the essential facts into one or two tight paragraphs and put them at the top of the chapter page, you could have characters refer to these events and expand upon them without needing a prologue or flashback. Frank Herbert was good at this, but not too many people are. But maybe give it a shot.

It would be like

"It is a well-known historical fact that blah blah blah, yet what has never been revealed is what so on and so forth really yadda yadda."
- From a newspaper article / Forgotten archive / Censored report within the secret order of so and so

and then dialogue could expand on it. Just an option.