Last Monday, Defense Secretary Jim Mattis, a retired Marine Corps general with a legendary appetite for military history, ticked off a list of book recommendations to a crowd of U.S. Army leaders and supporters—titles that might help them understand command, strategy and the ways war is evolving. But he kept coming back to one book in particular: T. R. Fehrenbach’s This Kind of War, a 54-year-old history of the Korean War that’s much better known in military than civilian quarters.