The Battle of Antietam was the bloodiest day in American history, and the turning point of the whole Civil War. At the point where Confederate troops invaded Maryland, poised to take Washington, the morale of the Union was at an all time low. It had suffered a series of defeats, it seemed that foreign governments were on the verge of recognizing the Confederacy and legitimizing its cause, and the Emancipation Proclamation had been shelved indefinitely. McPherson argues that the Union victory at Antietam sharply reversed all this, and gave the Union a new raison d'être in the form of Emancipation.

McPherson combines a compelling narrative of the battle itself with a clear analysis of the political situation surrounding it. The final chapters discuss the aftermath of the battle, and its reputation as a pivotal moment in American history.