In World War I, the 30th Infantry Division earned more Medals of Honor than any other American division. In World War II, it spent more consecutive days in combat than almost any other outfit. Recruited mainly from the Carolinas, Georgia and Tennessee, they were one of the hardest-fighting units the U.S. ever fielded in Europe. What was it about these men that made them so indomitable? They were tough and resilient for a start, but this division had...