What must citizens forget before a nation becomes a nation? Ethnic differences, for one thing: “No French citizen knows whether he is a Burgund, an Alain, a Taifala, or a Visigoth,” Renan said. Ancient differences as to sect or creed must be left in the past. “Every French citizen has forgotten,” Renan claims, that in the 13th century the pope’s armies nearly wiped out the Cathars, a rival Christian sect, and that on St. Bartholomew’s Day in the 16th century, Catholic mobs slaughtered thousands of Calvinist Protestants.

Happily, by Renan’s day, such old conflicts had fallen into time immemorial, and in so doing freed France to become France.

What can happen to a nation whose citizens do not forget? Renan would not have been surprised by the fate of the former Yugoslavia. For many years, Muslim Bosniaks, Roman Catholic Croats and Eastern Orthodox Serbs lived in relative harmony. With the collapse of Communism in the late 1980s, however, it seemed likely that the country would splinter into its constituent parts, and the Serbs, fearing they would become a second-class minority, began to massacre their Muslim neighbors.