The extreme right—neo-Nazis, the Ku Klux Klan and other assorted racists and anti-Semites—and the extreme left—anti-American and anti-Israel zealots, intolerant censors, violent anarchists such as Antifa, and other assorted radicals—both pose a danger in the U.S. and abroad.

Which group poses a greater threat? The question resists a quantitative answer, because much may depend on time and place. It may also be in the eye of the beholder: For many on the center left, the greater danger is posed by the hard right, and vice versa. Yet the most important reason for this lack of a definitive quantitative answer is that they pose qualitatively different dangers.

History has set limits on how far to the extremes of the hard right reasonable right-wingers are prepared to go. Following the horrors of the Holocaust and Southern lynchings, no one claiming the mantle of conservative is willing to be associated with Nazi anti-Semitism or the KKK. Neo-Nazi and Klan speakers are not invited to university campuses.

The hard left lacks comparable limits. Despite what Stalin, Mao, the Castros, Pol Pot, Hugo Chavez and North Korea’s Kims have done in the name of communism, there are still those on the left—including some university professors and students—who do not shrink from declaring themselves communists, or even Stalinists or Maoists. Their numbers are not high, but the mere fact that it is acceptable on campuses, even if not praiseworthy, to be identified with hard-left mass murderers, but not hard-right mass murderers, is telling.

The ultimate goals of the hard right are different, and far less commendable, than those of the hard left. The hard-right utopia might be a fascist society modeled on the Italy or Germany of the 1930s, or the segregationist post-Reconstruction American South.