On 3 October last year, the distinguished British-born historian Tony Judt was preparing for a public lecture when the telephone rang. He was due to give the talk, entitled 'The Israel Lobby and US Foreign Policy', at the Polish consulate in New York in less than an hour. The caterers were already there. But when he picked up the phone he was informed that his lecture had been suddenly cancelled.

He was also told that Abraham Foxman, the national director of the Anti-Defamation League (ADL) was on the phone to the Polish consul. Whether the call from the ADL was the cause of the cancellation would become the subject of heated debate in the days and months to come. Foxman labelled such accusations 'conspiratorial nonsense'; however, the Polish consul, Krzysztof Kasprzyk, later acknowledged that he had been contacted by a number of Jewish groups - including the ADL and the American Jewish Committee (AJC) - who were concerned about Judt's anti-Israel message.

'The phone calls were very elegant but may be interpreted as exercising a delicate pressure,' Kasprzyk said. It didn't take him long to see how it might look for Poland, given its history, to be fostering arguments that in certain spheres of American intellectual life have been conflated with anti-Semitism.

'They do what the more tactful members of the intelligence services used to do in late Communist society,' Tony Judt says of the ADL when I speak to him from his home in New York. 'They point out how foolish it is to associate with the wrong people. So they call up the Poles and they say: Did you know that Judt is a notorious critic of Israel, and therefore shading into or giving comfort to anti-Semites?'

In the New York Jewish press, the episode was dubbed - with a debatable degree of sarcasm - 'l'Affaire Judt'. Certainly, not everyone felt Judt was a latter-day Dreyfus. The New York Review of Books published an open letter to Abraham Foxman in Judt's defence, which was signed by 114 intellectuals, many of whom disagreed with Judt on the Middle East yet felt that his right to free speech had been indefensibly curbed. But Christopher Hitchens, reminiscing about an occasion when a talk of his own was cancelled for similar reasons, cried out: 'What a chance I missed to call attention to myself!' - not the sort of opportunity Hitchens is in the habit of passing up - 'Once again, absolutely conventional attacks on Israeli and US policy are presented as heroically original.'

In the past two weeks, the Judt Affair has entered an entirely new gear. In an essay written by the Holocaust scholar Alvin Rosenfeld and published by the American Jewish Committee, Judt's views - and those of other 'progressive Jews' such as the American playwright Tony Kushner and the British academic Jacqueline Rose - were expressly linked to anti-Semitism. That row was reported in the New York Times, giving it an unprecedented prominence, and since then the story has opened the floodgates of a debate that until now has been shrouded in fear. Americans have long been in the grip of a cultural taboo that is characterised by Judt as follows: 'All Jews are silenced by the requirement to be supportive of Israel, and all non-Jews are silenced by the fear of being thought anti-Semitic, and there is no conversation on the subject.'

Philip Weiss, a bold polemicist whose New York Observer blog, MondoWeiss, has been besieged by posts on the subject since he addressed it last week, has even gone so far as to declare a new movement. His account of it embraces the new forum for dissent, Independent Jewish Voices, which was launched in Britain last week by an eminent group that includes Eric Hobsbawm and Harold Pinter. In launching its manifesto, Independent Jewish Voices has taken the 40th anniversary of the occupation of the West Bank and Gaza Strip as an occasion to create 'a climate and a space in which Jews of different affiliations and persuasions can express their opinions about the actions of the Israeli government without being accused of disloyalty or being dismissed as self-hating.' One of its founding principles is: 'The battle against anti-Semitism is vital and is undermined whenever opposition to Israeli government policies is automatically branded as anti-Semitic.'

'A lot of people, like Tony Judt, have been doing brave work here in the US for a while,' Weiss tells me. 'What has happened specifically is that for once, the mainstream is paying attention.'

He dates the beginning of this back to last March, when an explosive article about the influence of the Israel lobby on American foreign policy, written by two American political scientists, Stephen Walt and John Mearsheimer, was published in the London Review of Books (having originally been turned down by the Atlantic Monthly). The response to the piece was so overwhelming - and so coloured by accusations of anti-Semitism - that the LRB decided to host a debate on the subject in New York last September. That debate was sold out; Tony Judt, one of the speakers, gave an exceptionally eloquent performance, in the course of which he said it was significant that the event had been hosted by a London publication. Public conversation on the issue had been so absent in America, he suggested, that it could only be opened up by importation.

'When Walt and Mearsheimer were published in London,' Philip Weiss continues, 'I said: something's changing.' Since then, the publication of former president Jimmy Carter's book, Palestine: Peace Not Apartheid, and the attention given to Rosenfeld's accusations in his AJC article, have proved, in Weiss's view, that 'there's no question that something has changed. One of the excitements of what's going on right now is that people who have had feelings about this and have not expressed them are popping up all over. It's personally very stirring to me that this is happening. I can't believe it.'

In fact, the debate is so current that the online magazine Slate has come up with a quiz entitled 'Are You A Liberal Anti-Semite?' (Sample question: 'Which state's offences against humanity bother you most? a) Sudan b) Israel c) Massachusetts'.) One of the prizes is dinner with Tony Judt.

Tony Judt is, in the words of a fellow historian, 'one of our most dazzling public intellectuals'. As a prominent professor at New York University and a regular contributor to the New York Review of Books, the New York Times and The Nation, he has a strong and widely heard voice. His latest book, Postwar - a magnificent, opinionated and vast history of Europe since 1945 - was voted one of the 10 best books of last year by the New York Times. A talented forger of links between thinkers from countries all over the world, Judt worked tirelessly after 1989 to bring together eastern European and American intellectuals, and he solidified these efforts by founding the Remarque Institute at NYU in 1995 to promote the study and discussion of Europe in America. A natural polemicist, he brought with him to New York an Oxbridge tradition more pugnacious than is generally characteristic of American academic life, and found himself - after years spent concentrating on European history - drawn back into an engagement with the Middle East.

In 2003, Judt wrote an articulately provocative piece for the New York Review of Books entitled 'Israel: The Alternative', in which he argued, among other things, that Israel was 'an anachronism' that was 'bad for the Jews' and should be converted into a binational state. The offices of the New York Review were inundated with letters as a result. Last year, Judt wrote an op-ed piece for the New York Times in which he argued that America's fear of anti-Semitism when discussing Israel wrought tremendous damage. As the page was about to go to press, the editor rang him up. 'Just one thing,' he said, 'You are Jewish, aren't you?'

Judt was born in London in 1948. Growing up Jewish in 1950s Britain, as he has said, he came to know a thing or two about anti-Semitism. His mother was from London and his father, who was born in Belgium, had come there as a stateless person. Judt was brought up in what he describes as 'a fairly standard left-wing Jewish secular political environment', but with close links to his Yiddish-speaking grandparents, all of whom were eastern European Jews, from Romania and Russia and Lithuania and Poland. As a teenager, he joined a left-wing Zionist organisation and became very active in the kibbutz movement, living in Israel on and off for a large part of the early 1960s.

'What changed for me,' he says now, 'was that in 1967 I went out as a volunteer at the time of the Six Day War; after the war was finished I volunteered for auxiliary military service and I ended up as a sort of informal translator for other volunteers up on the Golan Heights. And there for the first time I began to see another face of Israel that had been camouflaged from me by my enthusiasm for the idealism of the kibbutz movement.' He became, he recalls, quickly very detached from Israel. 'And in fact when I was a student in Paris I became involved in 1970 with Palestinians and young Israelis, trying to organise groups to talk about peace settlements and ending the conflict.'

Last week, as he looked over the list of signatories of the new British network, Independent Jewish Voices, Judt says he was struck by how many of them are people who have not in the past identified themselves publicly as Jewish. 'Of course they're Jewish,' he clarifies, 'but it was not part of their public identity tag. And now they feel - and I would share this sentiment - a need to say, look: if it helps you understand just how bad things have got in the Middle East, I am willing to act not as a freestanding historian but as a Jew. I don't normally like to act as though being Jewish was who I am, but it's a kind of inverse moral blackmail that forces you to go the other way.'

Speaking from Bloomington, Indiana, where he is a director of Indiana University's Jewish Studies Program, Alvin Rosenfeld tells me that his essay 'does seem to have struck a raw nerve'. 'I've been accused of wanting to shut down debate and stifle free speech,' he says, 'and none of that is true. I stand strongly for vigorous debate and open discussion. What in the past was said behind the hands and on the margins of society has been coming into the mainstream of discourse,' Rosenfeld adds, echoing the sentiments of those he attacks, 'Now one can deal with it. And that's one of the things I set out to do.'

Though Rosenfeld is careful not to say in his essay that anti-Zionism and anti-Semitism are identical, he does state that 'Anti-Zionism is the form that much of today's anti-Semitism takes, so much so that some now see earlier attempts to rid the world of Jews finding a parallel in present-day desires to get rid of the Jewish state.' He labels the work of Judt, Rose, Kushner et al 'This Jewish war against the Jewish state.' I ask him if he would say that an increase in anti-Zionist sentiment might be caused by Israeli policy. 'I doubt it,' he replies. 'As I read these people, it strikes to the heart, not of particular policies, but the idea of a sovereign Jewish state in the Middle East. I think it goes to the question of Israel's origins and essence.'

'Oh that's nuts,' Judt counters, 'I've never said Israel doesn't have a right to exist. I'm not actually sure that anyone in what we would call the respectable political mainstream ever has.'

'He says that,' says Rosenfeld, 'but it's not true. In his writings he calls not for a two-state solution but for the dissolution of the state of Israel and a one-state solution, and everyone knows that in no time at all, were such a scenario to come about, Jews would be a minority within this newly configured state, and would be at the mercy of a population that's not likely to treat them gently. Tony Judt is a kind of political fantasist, it strikes me.'

'The issue is not whether Israel has a right to exist,' Judt says plainly, 'Israel does exist. It exists just like Belgium or Kuwait or any other country which was invented at some point in the past and is now a fact. The question is what kind of a state Israel should be. That's all.'

Anti-Zionism has, like Zionism itself, a long and complicated history. 'The thing that we tend to forget,' Judt explains, 'is that until the Second World War, Zionism was a minority taste even within Jewish political organisations. The main body of European Jews was either apolitical or integrated, and voting within the existing countries they lived in. So to be anti-Zionist, at least until the late 1930s, was to be lined up with most Jews. It would make no sense to think of it as anti-Semitic.

'After the Second World War, for a fairly brief period - from let's say 1945 to about 1953 - the overwhelming majority of Jews who were politically thinking were Zionists, either actively or sympathetically, for the rather obvious reason that Israel was the only hope for Jewish survivors. But then many of them, like Hannah Arendt or Arthur Koestler, both of whom were Zionists at various points, took their distance, on the grounds that it was already clear to them that Israel was going to become the kind of state that as a cosmopolitan Jew they couldn't identify with.

'Ever since then, there has been an unbroken tradition of non-Israeli Jews who regard Israel as either unrelated to their own identity or something of which they sometimes approve, sometimes disapprove, sometimes totally dislike. This range of opinion is not new,' Judt concludes. 'The only thing that's new - and it's a product of the post-Sixties - is the insistence that it's anti-Semitic.'

Judt tells a story about an Israeli journalist who was in Washington in the 1960s. 'The Israeli ambassador was retiring, and the journalist asked him what he thought was his biggest achievement. The ambassador said: "I've succeeded in beginning to convince Americans that anti-Zionism is anti-Semitism." There has been a progressive emergence of a conflation,' Judt explains. 'It didn't just happen naturally. And it was pushed quite actively in the Seventies and Eighties, to the point at which it became so normal in this country that it was for a while the default assumption. It's really only in the last five to eight years that it's started to be questioned.'

The actions of very pro-Israel Jewish organisations - for instance, making carefully placed phone calls relating to certain public speakers - are, Judt believes, now born of panic rather than confidence.

'They've lost control of the debate,' he says. 'For a long time all they had to deal with were people like Norman Finkelstein or Noam Chomsky, who they could dismiss as loonies of the left. Now they're having to face, for want of a better cliché, the mainstream: people like me who have a fairly long established record of being Social Democrats (in the European sense) and certainly not on the crazy left on most issues, saying very critical things about Israel. They're not used to that, so their initial response has been to silence people if they could, and their second response has been to ratchet up the anti-Semitic charge.' Judt thinks it's telling that the New York Times 'is willing to report these issues and let reporters quote both sides. In the past, you would have had silence.'

Whether this will have any effect in Washington is another matter. The political influence of AIPAC (the pro-Israel lobby, American Israel Public Affairs Committee) is as strong as it ever was, and Judt argues that since it's not worth going out on a limb on Israel from Congressmen's point of view, change has to happen at a presidential level. Hillary, he says, 'is pretty gutless on this'; she has already given two gung-ho speeches to AIPAC. It's not a topic Barack Obama has yet picked up on, Judt adds, but Obama was brave enough to oppose the Iraq war from the outset, so it's possible that he would take a courageous stance elsewhere in the Middle East. 'A presidential candidate has to feel that once he or she gets into office - they wouldn't dare open their mouths while they're running for election - they don't stand to lose very much in public opinion if they put pressure on Israel,' Judt says

In Postwar, Judt writes of Europe that 'After 1989, nothing - not the future, not the present and above all not the past - would ever be the same.' Is there a moment like that, I ask him, in this situation? 'I think so,' he replies. 'It's not as tidy a moment as 1989 in Europe. But I think one could say that after the Iraq war, for want of a better defining moment, the American silence on the complexities and disasters of the Middle East was broken. The shell broke and conversation - however uncomfortable, however much slandered - became possible. I'm not sure that will change things in the Middle East, but it's changed the shape of things here. Even five years ago, I don't think it would have looked the way it does now.'

He sounds almost optimistic.

'Well,' he sighs, 'I do my best.'