Watch a preview of John Pilger's powerful new film, The War You Don't See, including interviews with Rageh Omaar and WikiLeaks founder Julian Assange Molinare

In the US Army manual on counterinsurgency, the American commander General David Petraeus describes Afghanistan as a "war of perception . . . conducted continuously using the news media". What really matters is not so much the day-to-day battles against the Taliban as the way the adventure is sold in America where "the media directly influence the attitude of key audiences". Reading this, I was reminded of the Venezuelan general who led a coup against the democratic government in 2002. "We had a secret weapon," he boasted. "We had the media, especially TV. You got to have the media."

Never has so much official energy been expended in ensuring journalists collude with the makers of rapacious wars which, say the media-friendly generals, are now "perpetual". In echoing the west's more verbose warlords, such as the waterboarding former US vice-president Dick Cheney, who predicated "50 years of war", they plan a state of permanent conflict wholly dependent on keeping at bay an enemy whose name they dare not speak: the public.

At Chicksands in Bedfordshire, the Ministry of Defence's psychological warfare (Psyops) establishment, media trainers devote themselves to the task, immersed in a jargon world of "information dominance", "asymmetric threats" and "cyberthreats". They share premises with those who teach the interrogation methods that have led to a public inquiry into British military torture in Iraq. Disinformation and the barbarity of colonial war have much in common.

Of course, only the jargon is new. In the opening sequence of my film, The War You Don't See, there is reference to a pre-WikiLeaks private conversation in December 1917 between David Lloyd George, Britain's prime minister during much of the first world war, and CP Scott, editor of the Manchester Guardian. "If people really knew the truth," the prime minister said, "the war would be stopped tomorrow. But of course they don't know, and can't know."

In the wake of this "war to end all wars", Edward Bernays, a confidante of President Woodrow Wilson, coined the term "public relations" as a euphemism for propaganda "which was given a bad name in the war". In his book, Propaganda (1928), Bernays described PR as "an invisible government which is the true ruling power in our country" thanks to "the intelligent manipulation of the masses". This was achieved by "false realities" and their adoption by the media. (One of Bernays's early successes was persuading women to smoke in public. By associating smoking with women's liberation, he achieved headlines that lauded cigarettes as "torches of freedom".)

I began to understand this as a young reporter during the American war in Vietnam. During my first assignment, I saw the results of the bombing of two villages and the use of Napalm B, which continues to burn beneath the skin; many of the victims were children; trees were festooned with body parts. The lament that "these unavoidable tragedies happen in wars" did not explain why virtually the entire population of South Vietnam was at grave risk from the forces of their declared "ally", the United States. PR terms like "pacification" and "collateral damage" became our currency. Almost no reporter used the word "invasion". "Involvement" and later "quagmire" became staples of a news vocabulary that recognised the killing of civilians merely as tragic mistakes and seldom questioned the good intentions of the invaders.

On the walls of the Saigon bureaus of major American news organisations were often displayed horrific photographs that were never published and rarely sent because it was said they were would "sensationalise" the war by upsetting readers and viewers and therefore were not "objective". The My Lai massacre in 1968 was not reported from Vietnam, even though a number of reporters knew about it (and other atrocities like it), but by a freelance in the US, Seymour Hersh. The cover of Newsweek magazine called it an "American tragedy", implying that the invaders were the victims: a purging theme enthusiastically taken up by Hollywood in movies such as The Deer Hunter and Platoon. The war was flawed and tragic, but the cause was essentially noble. Moreover, it was "lost" thanks to the irresponsibility of a hostile, uncensored media.

Although the opposite of the truth, such false realties became the "lessons" learned by the makers of present-day wars and by much of the media. Following Vietnam, "embedding" journalists became central to war policy on both sides of the Atlantic. With honourable exceptions, this succeeded, especially in the US. In March 2003, some 700 embedded reporters and camera crews accompanied the invading American forces in Iraq. Watch their excited reports, and it is the liberation of Europe all over again. The Iraqi people are distant, fleeting bit players; John Wayne had risen again.

A statue of Saddam Hussein is pulled down in Baghdad on 9 April 2003. Photograph: Jerome Delay/AP

The apogee was the victorious entry into Baghdad, and the TV pictures of crowds cheering the felling of a statue of Saddam Hussein. Behind this façade, an American Psyops team successfully manipulated what an ignored US army report describes as a "media circus [with] almost as many reporters as Iraqis". Rageh Omaar, who was there for the BBC, reported on the main evening news: "People have come out welcoming [the Americans], holding up V-signs. This is an image taking place across the whole of the Iraqi capital." In fact, across most of Iraq, largely unreported, the bloody conquest and destruction of a whole society was well under way.

In The War You Don't See, Omaar speaks with admirable frankness. "I didn't really do my job properly," he says. "I'd hold my hand up and say that one didn't press the most uncomfortable buttons hard enough." He describes how British military propaganda successfully manipulated coverage of the fall of Basra, which BBC News 24 reported as having fallen "17 times". This coverage, he says, was "a giant echo chamber".

The sheer magnitude of Iraqi suffering in the onslaught had little place in the news. Standing outside 10 Downing St, on the night of the invasion, Andrew Marr, then the BBC's political editor, declared, "[Tony Blair] said that they would be able to take Baghdad without a bloodbath and that in the end the Iraqis would be celebrating, and on both of those points he has been proved conclusively right . . ." I asked Marr for an interview, but received no reply. In studies of the television coverage by the University of Wales, Cardiff, and Media Tenor, the BBC's coverage was found to reflect overwhelmingly the government line and that reports of civilian suffering were relegated. Media Tenor places the BBC and America's CBS at the bottom of a league of western broadcasters in the time they allotted to opposition to the invasion. "I am perfectly open to the accusation that we were hoodwinked," said Jeremy Paxman, talking about Iraq's non-existent weapons of mass destruction to a group of students last year. "Clearly we were." As a highly paid professional broadcaster, he omitted to say why he was hoodwinked.

Dan Rather, who was the CBS news anchor for 24 years, was less reticent. "There was a fear in every newsroom in America," he told me, "a fear of losing your job . . . the fear of being stuck with some label, unpatriotic or otherwise." Rather says war has made "stenographers out of us" and that had journalists questioned the deceptions that led to the Iraq war, instead of amplifying them, the invasion would not have happened. This is a view now shared by a number of senior journalists I interviewed in the US.

In Britain, David Rose, whose Observer articles played a major part in falsely linking Saddam Hussein to al-Qaida and 9/11, gave me a courageous interview in which he said, "I can make no excuses . . . What happened [in Iraq] was a crime, a crime on a very large scale . . ."

"Does that make journalists accomplices?" I asked him.

"Yes . . . unwitting perhaps, but yes."

What is the value of journalists speaking like this? The answer is provided by the great reporter James Cameron, whose brave and revealing filmed report, made with Malcolm Aird, of the bombing of civilians in North Vietnam was banned by the BBC. "If we who are meant to find out what the bastards are up to, if we don't report what we find, if we don't speak up," he told me, "who's going to stop the whole bloody business happening again?"

Cameron could not have imagined a modern phenomenon such as WikiLeaks but he would have surely approved. In the current avalanche of official documents, especially those that describe the secret machinations that lead to war – such as the American mania over Iran – the failure of journalism is rarely noted. And perhaps the reason Julian Assange seems to excite such hostility among journalists serving a variety of "lobbies", those whom George Bush's press spokesman once called "complicit enablers", is that WikiLeaks and its truth-telling shames them. Why has the public had to wait for WikiLeaks to find out how great power really operates? As a leaked 2,000-page Ministry of Defence document reveals, the most effective journalists are those who are regarded in places of power not as embedded or clubbable, but as a "threat". This is the threat of real democracy, whose "currency", said Thomas Jefferson, is "free flowing information".

In my film, I asked Assange how WikiLeaks dealt with the draconian secrecy laws for which Britain is famous. "Well," he said, "when we look at the Official Secrets Act labelled documents, we see a statement that it is an offence to retain the information and it is an offence to destroy the information, so the only possible outcome is that we have to publish the information." These are extraordinary times.

www.johnpilger.com

• The War You Don't See is in cinemas and on DVD from 13 December, and is broadcast on ITV on 14 December at 10.35pm