(CNN)Many in the United States, including the President, are likely to welcome British Prime Minister Theresa May's suggestion that it is time to place restrictions on "safe space" Internet websites, such as Facebook and Google, that allegedly allow terrorist ideology to "breed."

Such a policy in the United States would clearly violate the First Amendment's sacred guarantee of free expression -- the very same principle that helped spur the American Revolution against British tyranny. This core American belief should not be tossed aside, even in the face of terrifying ISIS attacks.

The fight against "Islamist extremism" does require continued and even more aggressive military action in Great Britain and throughout the world, but that action should not take the form of restrictions on free political speech.

Such calls for censorship always emerge when terrorists, foreign and domestic, preach and kill in pursuit of their hateful ideology. Ironically the speech requiring the strongest defense is often the most hateful speech of all. But in these cases, those who believe in freedom must stand even more firmly. Otherwise all political expression will be in danger of censorship, depending on who runs the government at any point in time.

Defending hateful speech may appear to be a crazy academic or legal position until we look at the "slippery slope" toward fascist or socialist totalitarianism created when we adopt our own special bans on the free speech of others. Soon the ideological radicals are calling for their own "safe spaces" and censorship of what they define as "hate speech."

Those who oppose abortion as murder might seek to ban speech that advocates such "killings." "Right to choose" advocates on the other side of the argument might seek to ban anti-abortion talk as a form of gender-based discrimination. In the end, the free speech rights of all Americans would be determined by the ideological flavor of the party in power.

Once the censorship of political expression begins, everybody wants to impose his or her own particular definition of religious propriety, discrimination and political correctness. Opponents of the current majority view are then intimidated into silence by law and by the aggressively expressed moral or intellectual superiority of the majority. We are already seeing a lot of this on American college campuses.

UK PM raises terror threat level to critical 02:42

May's proposed Web regulation as well could lead to government regulation of the permissible parameters of Muslim faith discussions online. Those with "radical" tendencies beware. But what defines "radical" in the world of religion? Should the belief in Sharia law be banned as antithetical to the fundamental view that all religions are free to practice in a free secular democracy?

Does the advocacy of one of the diverse styles of Muslim female head-covering constitute a form of hateful gender-based discrimination that should be banned? Would the same ban apply to female head-covering by Roman Catholic women attending church on Sunday? This is where the "slippery slope" of speech censorship leads.

The nation's high court confirmed an important free speech doctrine that only "fighting words" can legitimately be banned without violating the First Amendment and that even the display of a Nazi swastika in a village occupied by numerous Holocaust survivors is permitted under the US Constitution. This forcefully confirmed prior precedent that banned speech must be akin to yelling "Fire!" in a crowded theater.

See British PM's full remarks on terror attack 08:00

In an age where overly sensitive college students and their supportive professors are seeking to ban unpopular speakers who advocate what they define as improper and "hateful speech," the Skokie case deserves to be added to the required reading list at American universities. There should be no "safe space" protection from free speech at any public forum in America, including college campuses and the Internet.

Intelligence agencies and law enforcement authorities have the right to review and monitor public Internet postings that suggest a direct link to ISIS' terror-related activities. Such sites may actually be helpful in locating and destroying ISIS terror cells.

If probable cause is established by the content of such postings, US law already provides the mechanism to follow up with a court-sanctioned search warrant and the arrest of a suspected conspirator.

What we do not need is an abridgment of our freedom of speech in a misguided effort to ensure the nation's security. We already fought one revolution to establish and preserve our First Amendment right, and we don't need another, prompted by the latest brand of barbarism and insanity emanating from the Middle East.

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Callan: We can't censor our way out of terrorism (Opinion) - CNN.com - CNN