The 19-year-old Marquis de Lafayette had met only a handful of Americans when he signed up to join General George Washington’s army, but he felt certain that the people of the United States were as honorable as the cause of freedom for which they fought. Their idealism was intoxicating, and its hold on Lafayette reminds us of a time when the young United States seemed to promise a brighter future for all mankind.

Lafayette was hardly the only Frenchman of his …