A British court last week sentenced four men to up to six years in prison for inciting murder and racial hatred. The men were among the hundreds of Muslims who in February of last year met after Friday prayers at London’s Regent’s Park mosque and marched to Denmark’s embassy to protest the Muhammad cartoons that had been published in a Danish newspaper. Like other such rallies, this protest descended into calls for terror and the beheadings of those who “insult Islam.”

The convicted men did nothing more destructive than shout, and in the view of some Britons, the judge went too far and dangerously curtailed the freedom of speech. But consider another view: By locking away the protestors for the words they chanted that day, the judge actually struck a victory for the freedom of speech.

Even in the Anglo-Saxon tradition, where freedom of expression has always known fewer limitations than in Continental Europe, it is not an absolute right. Laws regulating obscenity and hate speech have long set boundaries for what can be considered legitimate speech.

In this case, the slogans the men were chanting and had written on their placards were more than offensive; they were calls to mass murder. They were not vague threats that a society could afford to tolerate. Instead, by referring to actual acts of terror, the men’s words took on a concrete, menacing character. Among the threats they shouted were: “U.K. you will pay, 7/7 on its way,” “Oh Allah, we want to see another 9/11,” “Bomb, bomb Denmark, bomb, bomb U.S.A.” and “Bomb, bomb the U.K.”

The court’s judgment contrasts with the attitude of the police on hand that day. The protest took place just seven months after the July 7 London bombings, in which 52 people died. Yet the officers turned a blind eye—literally—to the protestors’ calls for more terror. They stood with their backs to the demonstrators, apparently ready to defend them against angry passers-by. British TV would later show an English bobby reprimanding a man who called for the protestors’ arrest. “You say one more thing like that, mate, and you’ll get yourself nicked (arrested),” the officer could be heard telling the onlooker. Public outrage about the police’s failure to stop this pro-terror rally prompted the later arrests of some of the protestors.

In his verdict, Judge Brian Barker said that “Freedom of speech and assembly have long been jealously guarded by our laws. . . . With freedom comes respect and responsibility, none of which was demonstrated by you.” He continued: “What you were part of was the complete opposite of peaceful protest. Your words were meant to foment hatred and encourage killing.”

Recall, this case started with a Danish newspaper’s decision to publish cartoons that satirized the Prophet Muhammad. The protestors’ goals were to impose their own standards of acceptable speech and silence dissenting voices. By striking down the demonstrators’“freedom” to intimidate and threaten, the court protected free speech for everybody else.

This is also what the Director of Public Prosecutions, Sir Ken MacDonald, suggested when he argued during the trial that glorifying the London subway bombings and calling for more terror “undermines everyone else’s freedom by stirring up bigotry, racial hatred and violence. Terror attacks our way of life and incitement can make a very real contribution to it.”

In fighting the war on terror, the challenge for Western societies is usually defined as finding the right balance between security and preserving the freedoms that make our democratic societies worth defending. But that’s not the whole story. Terrorists and their supporters threaten not just our lives; they also threaten our freedom—including our freedom of speech.

Some critics of radical Islam already live under constant police protection in fear of their lives. How many journalists or writers may mince their words out of fear of repercussions? The state has a duty not only to guarantee security, arguably its foremost task, but also to protect the open society against its Islamist enemies.

Civil libertarians’ almost reflexive objection to any type of tougher security measures stems from a noble tradition. History teaches members of democratic societies to see the state not so much as the guardian but as the natural enemy of civil liberties. That’s because liberty had to be gradually wrested from the hands of absolutist rulers. In Western political philosophy, freedom is defined as freedom from the state.

But there’s another danger too, one that we forget at our peril: If the state is not allowed to stop Islamists’ incitement to murder and terror, their speech may eventually be the only one that remains “free.”

Mr. Schwammenthal is an editorial writer for The Wall Street Journal Europe.