We know from history that the Union won the Civil War, but this novel puts that knowledge in a new perspective, showing the unfolding drama of the battle, the real horrors of those fights, and the painstaking deliberation and planning on both sides, as they make the best of bad situations.
The book doesn't openly celebrate the Union's point of view, except on the issue of slavery, as if to say, "For the purposes of studying the battles, let's ignore the politics." On slavery, though, the author does offer serious, thoughtful arguments about how slavery ended up being this contentious of an issue (in Chamberlain's voice). By reminding the reader that the issue of slavery is plain and simply, that it was evil and wrong, the reader is free to learn about both sides and their various strategies, in a more objective, detached way.
Instead of studying the philosophical differences between the sides, though, the novel shows the opposing armies like opposite sides in a chess game or something. The story is not about the ethics of either side's point of view, but rather, it is a straightforward depiction of the game of war, as played at Gettysburg by two teams of officers. The result is that the novel feels like a myth or something. It feels titanic in nature.