From Hendrickson Software Components:
Gettysburg is an ultra-realistic, historically accurate game set during the the Battle of Gettysburg, with a real GIS capability map including terrain data, ultra-realistic tactics that take into account the opposing unit's position, active troops, overall strength, fatigue, supply, morale, and the unit commander's command and control abilities. Fight the Battle of Gettysburg as the Army of Northern Virginia under the command of Robert E. Lee, or as the Army of the Potomac under the command of George G. Meade. The computer will play a historically accurate opposition. Will decisions that you make alter the outcome of the battle? Can the Army of Northern Virginia prevail?