On Jan. 2, 1864, Gen. Joseph Johnston, the commander of the largest Confederate army outside Virginia, called an evening meeting of his subordinate generals in his headquarters in northwest Georgia. The purpose of the meeting was kept secret from all but a few. When they arrived, they found Gen. Patrick Cleburne, an Irish immigrant and one of Johnston’s best division commanders, who presented a radical proposal: to enlist slaves as rebel soldiers in exchange for their freedom. Cleburne, who did not own slaves, focused on three points. First, the North had an insurmountable three-to-one manpower advantage among whites. Second, since...