How does political censorship work in liberal societies? When my film Year Zero: the Silent Death of Cambodia was banned in the United States in 1980, the broadcaster PBS cut all contact. Negotiations were ended abruptly; phone calls were not returned. Something had happened. But what? Year Zero had already alerted much of the world to Pol Pot's horrors, but it also investigated the critical role of the Nixon administration in the tyrant's rise to power and the devastation of Cambodia.

Six months later, a PBS official told me: "This wasn't censorship. We're into difficult political days in Washington. Your film would have given us problems with the Reagan administration. Sorry."

In Britain, the long war in Northern Ireland spawned a similar, deniable censorship. The journalist Liz Curtis compiled a list of more than 50 television films that were never shown or indefinitely delayed. The word "ban" was rarely used, and those responsible would invariably insist they believed in free speech.

The Lannan Foundation in Santa Fe, New Mexico, believes in free speech. The foundation's website says it is "dedicated to cultural freedom, diversity and creativity". Authors, film-makers and poets make their way to a sanctum of liberalism bankrolled by the billionaire Patrick Lannan in the tradition of Rockefeller and Ford.

The foundation also awards "grants" to America's liberal media, such as Free Speech TV, the Foundation for National Progress (publisher of the magazine Mother Jones), the Nation Institute and the TV and radio programme Democracy Now!. In Britain, it has been a supporter of the Martha Gellhorn Prize for Journalism, of which I am one of the judges. In 2008, Patrick Lannan backed Barack Obama's presidential campaign. According to the Santa Fe New Mexican, he is "devoted" to Obama.

World of not-knowing

On 15 June, I was due in Santa Fe, having been invited to share a platform with the distinguished American journalist David Barsamian. The foundation was also to host the US premiere of my new film, The War You Don't See, which investigates the false image-making of warmakers, especially Obama.

I was about to leave for Santa Fe when I received an email from the Lannan Foundation official organising my visit. The tone was incredulous. "Something has come up," she wrote. Patrick Lannan had called her and ordered all my events to be cancelled. "I have no idea what this is all about," she wrote.

Baffled, I asked that the premiere of my film be allowed to go ahead, as the US distribution largely depended on it. She repeated that "all" my events were cancelled, "and this includes the screening of your film". On the Lannan Foundation website, "cancelled" appeared across a picture of me. There was no explanation. None of my phone calls was returned, nor subsequent emails answered. A Kafka world of not-knowing descended.

The silence lasted a week until, under pressure from local media, the foundation put out a terse statement that too few tickets had been sold to make my visit "viable", and that "the Foundation regrets that the reason for the cancellation was not explained to Mr Pilger or to the public at the time the decision was made". Doubts were cast by a robust editorial in the Santa Fe New Mexican. The paper, which has long played a prominent role in promoting Lannan Foundation events, disclosed that my visit had been cancelled before the main advertising and previews were published. A full-page interview with me had to be pulled hurriedly. "Pilger and Barsamian could have expected closer to a packed 820-seat Lensic [arts centre]."

The manager of The Screen, the Santa Fe cinema that had been rented for the premiere, was called late at night and told to kill all his online promotion for my film. He was given no explanation, but took it on himself to reschedule the film for 23 June. It was a sell-out, with many people turned away. The idea that there was no public interest was demonstrably not true.

Symptom of suppression

Theories? There are many, but nothing is proven. For me, it is all reminiscent of long shadows cast during the cold war. "Something is going to surface," said Barsamian. "They can't keep the lid on this."

My 15 June talk was to have been about the collusion of American liberalism in a permanent state of war and in the demise of cherished freedoms, such as the right to call governments to account. In the US, as in Britain, serious dissent -- free speech -- has been substantially criminalised. Obama the black liberal, the PC exemplar, the marketing dream, is as much a warmonger as George W Bush. His score is six wars. Never in US presidential history has the White House prosecuted so many whistleblowers, yet this truth-telling, this exercise of true citizenship, is at the heart of America's constitutional First Amendment. Obama's greatest achievement is having seduced, co-opted and silenced much of liberal opinion in the US, including the anti-war movement.

The reaction to the cancellation has been illuminating. The brave, such as the great whistleblower Daniel Ellsberg, were appalled and said so. Similarly, many ordinary Americans called in to radio stations and have written to me, recognising a symptom of far greater suppression. But some exalted liberal voices have been affronted that I dared whisper the word censorship about such a beacon of "cultural freedom". The embarrassment of those who wish to point both ways is palpable. Others have pulled down the shutters and said nothing. Given their patron's ruthless show of power, it is understandable. For them, the Russian dissident poet Yevgeny Yevtushenko once wrote: "When truth is replaced by silence, the silence is a lie."