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. Slobodan Milosevic, a skilled apologist for the memory of difference, helped plant the seeds of that exercise in ethnic cleansing when he celebrated the anniversary of an ancient battle. For Serbs, it had become a so-called chosen trauma, an ancestral calamity whose memory mixes actual history with present-day grievance and hope. In the summer of 1389 at the Field of Blackbirds in Kosovo, an army of Muslim Turks defeated Christian Serbs led by the feudal lord Lazar Hrebeljanovic. The Ottoman Empire thereafter ruled over Kosovo for 400 years. To mark the 600th anniversary of that battle, Mr. Milosevic gathered a crowd on the original Field of Blackbirds. He himself descended on the site in a helicopter as if sainted Prince Lazar were returning from on high. Standing before the prominently displayed dates 1389 and 1989, he then delivered a speech whose constant references to dignity and humiliation, motherland and treason, bravery and suffering, pride and shame were clearly designed to provoke a time-collapsed modern re-enactment of the ancient animosity between Christians and Muslims. 
Henry Adams once called politics “the systematic organization of hatreds,” certainly the case in the former Yugoslavia. Not that American politicians have ever lacked such skills. One need only think of Ronald Reagan going to Neshoba County, Miss. — notorious site of the 1964 murders of three civil rights workers — to kick off his 1980 presidential campaign by telling a cheering crowd at the county fair, “I believe in states’ rights.” Or of Donald Trump Jr., who traveled to the same county fair in July 2016 to assure the crowd, “I believe in tradition,” a simple prompt in both cases for voters whose motivating “chosen trauma” is the Civil War and its legacy of racial animosity.