When it comes to history, we can be really certain about some things - especially when there is agreement in accounts written by people with different motives and perspectives, and even more so when contemporaries agree even though they are on opposite sides of the issue. Historical "occurrences" (what happened) are easier to analyze objectively (as difficult as even that is in some cases), but motivations and reasons for those occurrences are much harder to pin down with certainty in many cases.
That's true even with first-hand explanations. Eye-witness accounts are surprisingly shaky in many cases, as lawyers and police officers can attest. people aren't honest in some cases and, even when they are honest, they aren't always accurate. That last point is easy to forget:
Honest people can be wrong, especially if they write about something long after the fact - like much of our ancient scriptural records, for example (including the Book of Mormon) - or if their perspective colors their understanding.