I thought it was pretty bad, but one thing (well a couple things) really annoyed me.
Now, true, my only knowledge from the period of time this movie took place was from a man with a doctor of american history (in particular civil war era history), and another with a similiar level of qualification..but as far as I know.. this kind of thing never happened.
Also, the tactcs used by the lieutenant colonel (his name escapes me now) were just stupid. ANY union officer (or soldier for that matter) wouldn't have done any of the things he did.
And third, wheres the cavalry? Both Tom Cruise's character, and the lieutenant colonel were in the 7th cavalry in the American civil war!