On Feb. 14, 1989, I happened to be on a panel on press freedom for the Columbia Journalism Review when someone in the audience told us of Ayatollah Ruhollah Khomeini’s religious edict for blasphemy against the British novelist Salman Rushdie. What did we think? We didn’t, as I best recall, disgrace ourselves. We said most of the right things about defending freedom of thought and the imagination.

But the death sentence from Iran’s supreme leader seemed unreal—the sending of a thunderbolt from medieval Qom against modern Bloomsbury—and we didn’t treat it with the seriousness that it deserved. I recall, alas, making a very poor joke about literary deconstructionism. My colleagues, though more sensible, were baffled and hesitant. Was it even true—or perhaps just a mistranslation?

We knew soon enough that it was true. The literary, media and political worlds rallied in defense of Mr. Rushdie. He became a hero of free speech and a symbol—even if a slightly ambivalent postcolonial one—of Western liberal traditions. But he also went, very sensibly, behind a curtain of security that was to last many years.

And by degrees—when it seemed that not only Mr. Rushdie’s life but the lives of his publishers, editors and translators might be threatened—his base of support in the literary world thinned out. Sensitive intellectuals discovered that, in a multicultural world, respect for the Other meant understanding his traditions too, and these often were, well, sterner than ours. Freedom of speech was only one value to be set against…ahem, several other values. Fear, cowardice and rationalization spread outward.

Twenty-five years later, we can look back on a long series of similar events, including: the 2002 anti-Christian riots in Nigeria, in which more than 200 people were killed because a local tabloid had facetiously suggested that Miss World contestants would make suitable brides for Muhammad; the 2004 murder of the Dutch filmmaker Theo van Gogh for his movie “Submission,” in which passages from the Quran were printed on women’s bodies; the riots in Denmark and throughout the Middle East in 2005 in response to the publication of cartoons of Muhammad by a Danish magazine; the murder threats against Dutch politician Geert Wilders for his 2008 film “Fitna,” which interleaved passages from the Quran with clips of jihadist violence.

Muslim worshippers in Baghdad, Iraq, denounce Denmark after a Danish magazine ran cartoons of the Prophet Muhammad on Feb. 3, 2006. Associated Press

These events were threats to free speech, however, not only in themselves but also because they intimidated people and private organizations and gave governments an excuse to restrict free media. Over time, they encouraged others who had no interest in Islam whatsoever—from wealthy individuals to “dissident” minorities to democratic politicians—to try their hand at silencing opponents. Almost no newspapers published the Muhammad cartoons, for instance, though the story of them dominated the international media for weeks. Yale University Press especially distinguished itself by publishing a major study of the controversy in 2009—without the actual drawings.

Governments began to treat those threatened for their opinions almost as harshly as those attacking them. Dutch legal authorities tried repeatedly, if unsuccessfully, to prosecute Mr. Wilders for “inciting hatred” with his film. He was briefly prohibited from entering Britain. In 2006, Tony Blair’s government passed the Racial and Religious Hatred Act—a kind of “blasphemy lite” law—ostensibly designed to protect all religions against threatening expression but generally understood as intended to limit hostile criticism of Islam. Both the U.S. and the European Union have entered into a dialogue in recent years with the 56 states of the Organization of the Islamic Conference, which is seeking an international law prohibiting blasphemy. In 2011, Secretary of State Hillary Clinton told the OIC that, while the First Amendment prevented the U.S. from prohibiting speech, the administration might still “use some old-fashioned techniques of peer pressure and shaming so that people don’t feel they have the support to do what we abhor.”

Admittedly, it is difficult to draw a clear line between criticism of an Islamic belief and an attack on Muslims who believe it. If you denounce a belief as absurd, you are implicitly criticizing the believers as credulous fools. Christians have to endure explicit denunciations of their faith all the time from such writers as Richard Dawkins and Sam Harris. And so they should. If you can’t stand the heat, don’t listen to hellfire sermons from atheists.

Hearing criticisms of your own convictions and learning the beliefs of others are training for life in a multifaith society. Preventing open debate means that all believers, including atheists, remain in the prison of unconsidered opinion. The right to be offended, which is the other side of free speech, is therefore a genuine right. True belief and honest doubt are both impossible without it.

It isn’t just some Muslims who want the false comfort of censoring disagreeable opinions. Far from it. Gays, Christians, feminists, patriots, foreign despots, ethnic activists—or organizations claiming to speak for them—are among the many groups seeking relief from the criticism of others through the courts, the legislatures and the public square.

England’s libel laws—long a scandalous system for enabling the rich to suppress their scandals—now have imitations in Europe and the U.S. In May 2014, the European Court of Justice created “the right to be forgotten,” enabling those with ugly pasts—a fraudster, a failed politician, an anti-Muslim bigot perhaps—to delete their crimes, misdemeanors and embarrassments from Internet records so that search engines cannot find them.

Surely such things can’t happen in the land of the First Amendment? Not in quite the same way, perhaps, but a libel suit brought by the climatologist Michael Mann against the opinion writer Mark Steyn, National Review magazine (with which I am affiliated) and the Competitive Enterprise Institute for their criticism of his temperature projections still poses a chilling threat to free speech and scientific debate. Even if the case is ultimately resolved in favor of Mr. Mann’s critics, they will have suffered a considerable loss in time and money. “The process is the punishment,” Mr. Steyn has said of such trials. It is also a deterrent to future critics.

Nor are conservatives free from sin on this issue. In recent years, their attacks on free expression in the U.S. have generally been prompted by a philistine discomfort with provocative art, from the “Sensation” exhibition at the Brooklyn Museum in 1999 to the more recent flap over “The Death of Klinghoffer” at New York’s Metropolitan Opera.

In Britain, the sitting Tory home secretary, Theresa May, long resisted efforts to reform a catchall law regulating speech that the police have enforced with extraordinary zeal and no sense of proportion. These police actions include arresting a protester for asking a policeman “Is your horse gay?”; prosecuting a drunken soccer fan who, from his sofa, attacked a player in a racist tweet; summoning a youngster to appear in court for a placard describing Scientology as a cult; and arresting a Muslim demonstrator for burning a Remembrance Poppy.

Under the new British law, an evangelical Christian also was fined for holding up a sign that read “Stop Homosexuality, Stop Lesbianism.” But he was lucky. A human rights tribunal in Canada imposed a lifetime ban on sermonizing about homosexuality on a clergyman who had similarly offended. In both countries, the restraints on speech have since been softened, but the concessions have been modest, and Canada’s Supreme Court has clearly indicated a wish to retain the new speech regime in full.

This slow erosion of freedom of expression has come about in ways both social and legal. Before the 1960s, arguments for censorship tended to focus on sexual morality, pornography and obscenity. The censors themselves were usually depicted as benighted moral conservatives—priggish maiden aunts. Freedom of political speech, however, was regarded as sacrosanct by all. As legal restraints on obscenity fell away, however, freedom of political speech began to come under attack from a different kind of censor—college administrators, ethnic-grievance groups, gay and feminist advocates.

A woman protests a speech by Geert Wilders in Australia in 2013. Mr. Wilders’s 2008 film ‘Fitna’ interweaved passages from the Quran with clips of jihadist violence. Agence France-Presse/Getty Images

The new censors advanced such arguments as that “free speech can never be an excuse for racism.” These arguments are essentially exercises both in begging the question and in confusing it. While the principle of free speech cannot justify racism any more than it can disprove racism, it is the only principle that can allow us to judge whether or not particular speech is racist. Thus the censor’s argument should be reversed: “Accusations of racism can never be an excuse for prohibiting free speech.”

Meanwhile, the narrowly legal grounds for restricting speech changed, too. Since the 18th century, the basic legal justifications for restricting political speech and publication were direct incitement to harm, national security, maintaining public order, libel, etc. Content wasn’t supposed to be considered (though it was sometimes smuggled in under other headings).

Today, content is increasingly the explicit justification for restricting speech. The argument used, especially in colleges, is that “words hurt.” Thus, universities, parliaments, courts and various international bodies intervene promiscuously to restrict hurtful or offensive speech—with the results described above. In the new climate, hurtful speech is much more likely to be political speech than obscene speech.

The definition of political speech has changed too. The U.S. Supreme Court has expanded it to include nonspeech actions, such as nude dancing. Conservative judges such as the late Robert Bork had some fun pointing out that, under the Court’s rulings, students couldn’t lawfully pray before a football game, but they might dance naked—unless the dance included scarves, since this might constitute a prohibited “biblical” allusion to Salome’s veils.

There is plainly scope for disagreement over what actions amount to protected speech, but in a landmark ruling in 1989, the Supreme Court decided that burning the American flag was among them. If one accepts the notion of nonspeech opinion, it is difficult to disagree. Burning the flag of the U.S., however odious, is plainly a statement meant to reject American power and legitimacy.

The Court’s judgment in this case was the apogee of post-Vietnam free speech liberalism. But to bring the issue up-to-date: Burning the Quran also expresses a political opinion. Would today’s justices reach the same conclusion as their predecessors did on the question of flag-burning? Would a U.S. administration that is discussing an international blasphemy law with the world’s Islamic states tell the Court that burning a Quran is also protected speech? It is, at the very least, doubtful.

We know that today’s British government wouldn’t do so. Burning a Remembrance Day poppy is the British equivalent of flag-burning, and Ms. May, the home secretary, implicitly disagreed with the American precedent in her defense of Britain’s restrictive laws. She explained that “a careful balance [had] to be struck between protecting our proud tradition of free speech and taking action against those who cause widespread offense.” Realistically parsed, this amounts to an acknowledgment that Britain’s proud tradition of free speech will not in fact be protected. Speech that offends significant groups of people—as distinguished from speech that directly invites or threatens violence—will be censored or punished.

Some years ago, the liberal writer Michael Kinsley described the different attitudes to free speech in the U.K. and the U.S. as follows: “In a country like Great Britain, the legal protections for speech are weaker than ours, but the social protections are stronger. They lack a First Amendment, but they have thicker skin and a greater acceptance of eccentricity of all sorts.”

Today, both sorts of protection for speech—legal and social—are weaker than before in both countries. This year, official regulation of the press was passed into U.K. law for the first time since 18th-century juries nullified press prosecutions. These new restraints enjoyed the backing not just of all the parties but apparently of the public as well.

In the U.S., the case of Mann v. Steyn, let alone a hypothetical case involving Quran-burning, has yet to be decided. But Democrats in the Senate are seeking to restrict political speech by restricting the money spent to promote it. And in the private sector, American corporations have blacklisted employees for expressing or financing certain unfashionable opinions. In short, a public culture that used to be liberal is now “progressive”—which is something like liberalism minus its commitment to freedom.

The U.S. and Britain have long thought of themselves as, above all, free countries. If that identity continues to atrophy, free speech will be the first victim. But it will not be the last.

--Mr. O’Sullivan is director of the Danube Institute in Budapest and a senior fellow of the National Review Institute in New York.