In the new normal of 2017, in which far-right and far-left militants clash openly in America’s public square, a contentious 20th century debate is newly relevant: is the extreme left as dangerous and repugnant as the extreme right? Should the hammer and sickle be as offensive as the swastika? Was Communism as evil as Nazism — a question sure to generate plenty of heat as we approach the 100th anniversary of the Russian revolution? Conservatives have long complained of a double standard for Nazi and Communist crimes. To manyon theleft, on the other hand, equating Communism and Nazism is an...