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.

Fehrenbach, Mattis explained during his address to the Association of the United States Army’s Exposition on Building Readiness, reminds us of two essential truths about war: its “primitive, atavistic, and unrelenting nature” and the “absolutely fundamental” importance of boots on the ground, even in an age of drone attacks and cyberwarfare. “You may fly over a nation forever, you may bomb it, atomize it, pulverize it and wipe it clean of life. But if you desire to defend it, if you desire to protect it, if you desire to keep it for civilization, you must do this on the ground the way the Roman legions did: by putting your young men in the mud,” Mattis said, quoting Fehrenbach. “I would only modify it today by saying, ‘by putting your young men and women in the mud.’”

These citations were preludes to Mattis’s final punch. The last question he took from the audience finally addressed the concern on everyone’s mind: As Trump and Kim Jong Un exchange dire nuclear threats, “what can the U.S. military do to lessen the likelihood of conflict on the Korean Peninsula?”