In the autumn of 1862, Robert E. Lee took the war north into Maryland, hoping to shake the Union and win foreign recognition for the Confederacy. What he did not count on was losing his battle plan, wrapped around a set of cigars, and having it fall into George McClellan’s hands. That stroke of luck set the stage for a clash in the mountain passes above the Maryland countryside.

On September 14, the Union Army fought its way through Crampton’s, Fox’s, and Turner’s Gaps in the Battle of South Mountain. The fighting was brutal, the casualties high, and two future presidents, Rutherford Hayes and William McKinley, were in the thick of it. South Mountain is often overshadowed by Antietam, fought just three days later, but it was a pivotal Union victory. It showed that Lee could be stopped, and it gave Lincoln the opening to redefine the war.