One hundred years ago, by the Spring of 1915, a full two years before America’s declaration of war against Germany, hundreds of young American men were already serving in or near the front lines— as infantrymen in the French Foreign Legion, as aviators in the Lafayette Flying Corps or Lafayette Escadrille, or as ambulance drivers with the American Field Service. These young men, many of them Ivy Leaguers from Harvard, Yale and Princeton, and other top universities, joined up of their own accord and at their own expense, putting their lives on hold for several years, for the privilege of defending someone else’s country. Their motives were both idealistic and personal, involving a love of French culture and the French generally, a hatred for what they saw as Teutonic militarism and aggression, and a degree of shame and impatience toward their own country, for failing to rise to what they…