Obviously, you can't go into details about how and why the history of the time happens. Not unless you're writing a book that will replace Sominex for insomniacs.

But, how much history do you need? Is it enough to say your hero is marching into Manassas, serving under Longstreet, ready to send the BlueBellies scampering back to Washington, and assume you reader will know this must be the first battle of Bull Run in the US Civil War? Or do you need to explain that this is the first battle of the war, both sides are ill-prepared, overconfident and flat out stupid?

Well, any story about that battle will prove the stupid part, but that's not my point.

The question is how do you let the reader know this, and how much can you assume they know.

The answer to how much you can assume is "Next to Nothing". Unfortunately, while you need to educate the clueless reader, you can't bore the avid history readers who know more about this event than you do.

We agreed that the history needs to be woven into the story, not bunched into an infodump someplace, and it has to be presented in an interesting manner.

And you should avoid the dreaded "As you know, Bob..." info-dump disguised as dialog (or worse, monolog).