In early July 1864, a quickly patched together force of outnumbered Union soldiers under the command of Maj. Gen. Lew Wallace prepared for a last-ditch defense along the banks of the Monocacy River. Behind them, barely fifty miles away, lay the capital of the United States, open to attack. Facing Wallace's men were Lt. Gen. Jubal Early's Confederates. In just under a month, they had cleared the Shenandoah Valley of Union soldiers and crossed the Potomac River, invading the north for the third time in the war. The veterans in Early's force could almost imagine their flags flying above the...