Although it is considered a legacy title now that Civil War II has been released (see Narko's link), you might want to consider its predecessor AGEOD's American Civil War:
http://www.a-acw.com/ (Use the Wayback Machine to view at
http://archive.org/ )
http://www.ageod-forum.com/forumdisp...ican-Civil-War
http://ageod.nsen.ch/aacwwiki/
A demo is available for the game. You might be able to find AGEOD bundles such as AGEOD's Military Strategies: Battles of 1750-1918 that include physical copies of the game as well
Good luck!