Great historical fiction should, in my opinion, create a resonant reflexivity whereby the reader's knowledge of the historical context enriches the fiction and the fiction of the book helps one to imagine, or reimagine, the history. Otherwise, the history is simply ambiance, a somewhat arbitrary setting. This achieves greatness, not least of all by humanizing the men leading the south. It's all too easy to accept the moral clarity that comes from victory or the grand view, but being embedded in the moment you realize that it's the same sun rising and falling on the north and south for those three days of Gettysburgh.
As a side note, I'm convinced this was one of THE key models for George RR Martin in writing Game of Thrones. If you love Game of Thrones, definitely give this a read.