&gt; “Never before in our history have enemies outside the United States been able to propagate genuinely dangerous ideas on American territory in such an effective way,” Mr. Posner wrote. The Islamic State’s ability to spread “ideas that lead directly to terrorist attacks,” he said, “calls for new thinking about limits on freedom of speech.”<p>This seems to me like the keystone of the argument, and I&#x27;m fairly sure a good bout of research into anarchist and communist terrorism&#x2F;propaganda around the turn of the 20th century would show it false.