The problem is that exceptionalism leads to bad things. The first is hypocrisy. How, for instance, could the U.S. or U.K. ever have claimed to be morally superior when the first English ship carrying African slaves to America arrived in 1619, a year before that other English ship, the Mayflower, brought the Pilgrims to their city upon a hill? And what would either country say if the anti-racism riots of recent weeks — late blowback for that earlier legacy — had taken place in, say, China or Iran? Exceptionalism requires editing a country’s past, and indeed lying.
It also leads to double standards. In the American case, it often becomes “exemptionalism,” when the U.S. doesn’t feel bound by international treaties or courts, even as it criticizes other countries for falling foul of them. Such arrogance provokes resentment and conflict.
In the worst cases, such as Germany’s or Japan’s during the past century, exceptionalism mutates into a brutish ethnocentrism that leads to atrocities, tragedy and ruin. That’s why the word “Sonderweg” has acquired an entirely negative connotation among historians in postwar Germany, as a delusion that culminated in the Holocaust.