You've got the wrong man: Anti-fascist organisations pursue UKIP but let Islamist extremists off the hook (photo: Christopher Furlong/Getty Images)

The state intervenes when the principles of a liberal society collapse. Usually it blunders in. Invariably it destroys basic freedoms. No one except the most blinkered supporters of authoritarian government can predict with confidence that its "crackdowns" and "emergency measures" will make our lives better or safer. But there you are. When supposedly good and responsible people fail to police themselves, the government will summon the real police to do the job for them.

For years, a dizzying gulf has stretched between the principles most good and responsible liberals say they hold — beliefs in reasoned argument, democracy, and equal rights for women, gays and people of all colours and creeds — and their practical failure to oppose radical Islam. A few of us tried to persuade them to mean what they say and behave accordingly. Some of us have stayed on the Left. Others have given up on what looks an irredeemably compromised movement and attacked liberal-left orthodoxy from the right. I will not pretend that any of us have had a great deal of success.

"A new scientific truth does not triumph by convincing its opponents and making them see the light," said Max Planck, "but rather because its opponents eventually die, and a new generation grows up that is familiar with it."

I thought that liberal values would only creep forward at the Planckian pace of "one funeral at a time", and we would have to wait for the current generation of liberal-leftists to die out before we saw progress. I forgot that outsiders can impose changes insiders refuse to contemplate. Whatever their politics, religion or occupation, people who live in bubbles confuse their world with the whole world. Lost in their little rituals and taboos, they do not see that others are noting their faults and weighing whether or not to interfere.

The schools can seem a self-enclosed system. Meet an educationalist and he or she will be on the Left nine times out of ten. Yet when fanatics from the Islamist religious Right took over Birmingham schools, the teaching unions, Labour councillors and the liberal press did everything they could to cover up a plan to impose a reactionary education on British children. Unions did not defend secular teachers when hard-line governors forced them out. Supposed leftists did not worry that governors were making a "sustained and coordinated" effort "to force the segregationist attitudes and practices of a hard-line and politicised strain of Sunni Islam" on British boys and girls, as Birmingham City Council's inquiry put it. Right until the moment when they could no longer deny the truth, they claimed the scandal was an Islamophobic plot, manufactured by Conservatives trying to exploit racial tensions.

It has been that way for what feels like forever in Britain's schools. You don't tackle difficult subjects. You never lay yourself open to accusations of racism, however spurious and self-interested they may be. It felt it could carry on that way forever too. But all of a sudden the state, and I suspect the public, decided it had had enough. Government inspectors put the Sir John Cass Church of England school under "special measures" a few weeks ago. Its teachers and governors will face extraordinary scrutiny and can be dismissed at short notice, not because they were plotting to indoctrinate children — they are by all accounts admirable people running an admirable school — but because they failed to spot that Muslim pupils were linking to extremist sites on Facebook.

For what would have been a forgivable lapse only last year, a good school has lost its autonomy. Every school in the country now knows it must treat radical Islam in much the same way as it treats extreme white racism or suffer. The liberal failure to be honest about political Islam in Birmingham is having national consequences.