Douglas Giles is a recent casualty. He used to teach a class on world religions at Roosevelt University, Chicago, founded in memory of FDR and his liberal-inclined wife, Eleanor. Last year, Giles was ordered by his head of department, art historian Susan Weininger, not to allow students to ask questions about Palestine and Israel; in fact, nothing was to be mentioned in class, textbooks and examinations that could possibly open Judaism to criticism.

Students, being what they are, did not go along with the ban. A young woman, originally from Pakistan, asked a question about Palestinian rights. Someone complained and Professor Giles was promptly fired.

Leaving aside his boss's doubtful qualifications to set limits on a class of comparative religion - her speciality is early 20th-century Midwestern artists such as Tunis Ponsen (nor have I) - the point to grasp is that Professor Giles did not make inflammatory statements himself: he merely refused to limit debate among the young minds in front of him.

This might be seen as a troubling one-off like the story involving the president of Harvard, Lawrence Summers, who suggested that innate differences between the minds of men and women could be one reason why fewer women succeed in science and maths careers and was then ousted. But Giles's sacking is far more important because it is part of the movement to suppress criticism of Israel on the grounds that it is anti-semitic. A mild man, Giles seems astonished to find the battle for free speech in his own lecture theatre.

'It may be sexy to get on a bus and go to DC and march against war,' he said to me last week. 'It is much less sexy to fight in your own university for the right of free speech. But that is where it begins. That is because they are taking away what you can talk about.' He feels there is a pattern of intolerance in his sacking that has been encouraged by websites such as FrontPageMag.com and Campus Watch.

Joel Beinin of Stanford University is regularly attacked by both. Beinin is a Jew who speaks both Hebrew and Arabic. He worked in Israel and on an assembly line in the US, where he helped Arab workers understand their rights. Now, he holds seminars at Stanford in which all views are expressed. For this reason, no doubt, his photograph recently appeared on the front of a booklet entitled 'Campus Support for Terrorism'.

It was published by David Horovitz, the founder of FrontPageMag.com who has both composed a bill of rights for universities, designed to take politics (for which read liberal influence and plurality) out of the curriculum and a list of the 100 most dangerous academics in America, which includes Noam Chomsky and many other distinguished thinkers and teachers.

The demented, bullying tone of the websites is another symptom of the descent of public discourse in America and, frankly, one can easily see the attractions of self-censorship on the question of Middle East and Israel. Read David Horovitz for longer than five minutes and you begin to hear Senator Joseph McCarthy accusing someone of un-American activities.

At Harvard, a few weeks after what was called Summers's 'mis-step', a much greater row ensued when John Mearsheimer of the University of Chicago and Stephen Walt of Harvard published a paper called 'The Israel Lobby'. Brave because the alleged distortion of US pro-Israel foreign policy is unmentionable in American public life.

Their paper was printed only in the UK, in the London Review of Books. In America, there then followed what has been described as the massive 'Shhhhhhhhh!' Apart from the mud-slinging from sites such as Campus Watch and FrontPageMag, it has had little mainstream circulation and there has been no real debate.

I have read it several times and cannot disagree with an early point made by the authors. 'There is a strong moral case for supporting Israel's existence, but that is not in jeopardy. Viewed objectively, its past and present conduct offers no moral basis for privileging it over the Palestinians.' That is the crux. All Americans, to say little of the British who have been reluctantly welded to US policy, surely deserve the chance to know about the influence that lobbies such as the American-Israeli Public Affairs Committee (AIPAC) exert at times like these.

'The bottom line,' say Mearsheimer and Walt, 'is that AIPAC is a de facto agent for a foreign government, has a stranglehold on Congress, with the result that US policy is not debated there, even though the policy has important consequences for the entire world. In other words, one of the three main branches of the government is firmly committed to supporting Israel.'

Later they say: 'The lobby's influence causes trouble on several fronts. It increases the terrorist danger that all states face, including America's European allies. It has made it impossible to end the Israeli-Palestinian conflict, a situation that gives extremists a powerful recruiting tool, increases the pool of potential terrorist and sympathisers and contributes to Islamic radicalism in Europe and Asia.'

You could add that the lobby's influence may, in the long run, be very much against Israel's interests.

That is my belief, but these things are rarely discussed in America. People look vaguely queasy when you raise the subject of the Israeli lobby, as though the only concern in American discourse is not to appear anti-semitic, a fear which, I suggest, is sometimes shamelessly played upon.

The right of people like Mearsheimer, Walt, Beinin, Giles and even Summers to say what they think must remain inviolate if we are not to lose the values the West insists its fighting for. A little boldness is called for on both sides of the Atlantic to question the pressure coming from both Jewish and Muslim quarters not to discuss issues openly because of various sensibilities.

In Britain, we should deplore with equal vehemence the temptation to give into special pleading from, for instance, the Muslim businessmen who do not want the film of Monica Ali's Brick Lane made in their area. They have no right to dictate to this ancient democracy of ours - now theirs - and so stifle free expression.

Last week, during Jon Snow's fascinating Channel 4 documentary about Muslim attitudes in this country, a woman said that British society was too decadent for her to allow her children to integrate completely. A moment's thought suggested that British democracy had much to offer over the appalling civic values found in most Muslim countries, the oppression of women in Islam, the untold domestic abuse and the tens of thousands of children sold into bonded labour in Pakistan - her husband's country of origin. Her prim separatism fails to grasp the value of our democratic institutions when set against societies run by Sharia law and so undermines them.

My view is that in America and Britain, we should think of free speech as an article of faith, as one of the ways that we define our civilisation against the forces that were to be unleashed on us this week, as well as the influences that stifle criticism of Israel and so enable the disgraceful actions in south Lebanon.

The interests of extreme proponents of Muslim and Jewish faiths combine in one way or another to assault our ancient democratic traditions and we must resist them.

Let the students like those in Douglas Giles's class ask whatever they like.