Late in the afternoon of September 17, roughly an hour before the sunset that would mark the close of the bloodiest day in U.S. history, a Maine regiment met with a wholly unnecessary fate. Just when the regiments’ soldiers thought they’d made it through the battle relatively unscathed, they got pulled back in with disastrous consequences. At Antietam, due to Union commander George McClellan’s piecemeal strategy, the fighting was conducted sector by sector. (General Phil Kearny once described McClellan as “fighting by driblets.”) The severe topography of the Antietam Valley, featuring a field carved into discrete sections, contributed further to...