For much of this year, a display in the headquarters of Lt. Gen. Ben Hodges, the commander of U.S. Army forces in Europe, featured a quote from a German admiral predicting America would never enter World War I.

“This is what failed deterrence looks like,” Gen. Hodges said recently as he glanced at the quote at his base in Wiesbaden, Germany. His goal during his three years in his post has been to make sure any adversary of the U.S. and its allies has no excuse for making such a dangerous miscalculation today.

Gen....