By affirming benign goals, Antifa and its comrades make intolerance and even violence seductive.

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 danger posed by the extreme left is directly related to its more benign goals, which seduce some people, including university students and faculty. Believing that noble ends justify ignoble means, they are willing to accept the antidemocratic, intolerant and sometimes violent censorship policies and actions of Antifa and its radical cohorts. For that reason, the most extreme left zealots are welcomed today on many campuses to express their radical views. That is not true of the most extreme neo-Nazi or KKK zealots, such as David Duke and Richard Spencer. Former White House aide Steve Bannon recently told “60 Minutes” that “the neo-Nazis and neo-Confederates and the Klan, who by the way are absolutely awful—there’s no room in American politics for that.” In contrast, prominent American leftists, such as Noam Chomsky and even Bernie Sanders, supported the candidacy of British hard-left extremist Jeremy Corbyn, despite his flirtation with anti-Semitism.

The hard right is dangerous largely for what it has done in the past.

The danger posed by the extreme hard left is more about the future. Leaders of tomorrow are being educated today on campus. The tolerance for censorship and even violence to suppress dissenting voices may be a foretaste of things to come. The growing influence of “intersectionality”—which creates alliances among “oppressed” groups—has led to a strange acceptance by much of the extreme left of the far-from-progressive goals and violent means of radical Islamic terrorist groups that are sexist, homophobic, anti-Semitic and anti-Western. This combination of hard-left secular views and extreme Islamic theological views is toxic. We must recognize the different dangers posed by different extremist groups that preach and practice violence, if we are to combat them effectively in the marketplace of ideas, and perhaps more importantly, on the campuses and streets.

More From The Wall Street Journal (subscription required):