The way wars are reported in the western media follows a depressingly predictable pattern: stage one, the crisis; stage two, the demonisation of the enemy's leader; stage three, the demonisation of the enemy as individuals; and stage four, atrocities. At the moment we are at stages two and three: efforts to show that not only Osama bin Laden and the Taliban are fanatical and cruel but that most Afghans - even many Muslims - are as well. We are already through stage one, the reporting of a crisis which negotiations appear unable to resolve. Politicians, while calling for diplomacy, warn of military retaliation. The media reports this as "We're on the brink of war", or "War is inevitable".

News coverage concentrates on the build up of military force, and prominent columnists and newspaper editorials urge war. But there are usually sizable minorities of citizens concerned that all avenues for peace have not been fully explored and although the mainstream media ignores or plays down their protests, these have to be dampened down unless they gain strength.

We now enter stage two of the pattern - the demonisation of the enemy's leader. Comparing the leader with Hitler is a good start because of the instant images that Hitler's name provokes. So when George Bush Sr likened Iraq's takeover of Kuwait with the Nazi blitzkrieg in Europe in the 1930s, the media quickly took up the theme. Saddam Hussein was painted as a second Hitler, hated by his own people and despised in the Arab world. Equally, in the Kosovo conflict, the Serbs were portrayed as Nazi thugs intent on genocide and words like "Auschwitz-style furnaces" and "Holocaust" were used.

The crudest approach is to suggest that the leader is insane. Saddam Hussein was "a deranged psychopath", Milosevic was mad, and the Spectator recently headlined an article on Osama bin Laden: "Inside the mind of the maniac". Those who publicly question any of this can expect an even stronger burst of abuse. In the Gulf war they were labelled "friends of terrorists, ranters, nutty, hypocrites, animals, barbarians, mad, traitors, unhinged, appeasers and apologists". The Mirror called peace demonstrators "misguided, twisted individuals always eager to comfort and support any country but their own. They are a danger to all us - the enemy within." Columnist Christopher Hitchens, in last week's Spectator article, Damn the doves, says that intellectuals who seek to understand the new enemy are no friends of peace, democracy or human life.

The third stage in the pattern is the demonisation not only of the leader but of his people. The simplest way of doing this is the atrocity story. The problem is that although many atrocity stories are true - after all, war itself is an atrocity - many are not.

Take the Kuwaiti babies story. Its origins go back to the first world war when British propaganda accused the Germans of tossing Belgian babies into the air and catching them on their bayonets. Dusted off and updated for the Gulf war, this version had Iraqi soldiers bursting into a modern Kuwaiti hospital, finding the premature babies ward and then tossing the babies out of incubators so that the incubators could be sent back to Iraq.

The story, improbable from the start, was first reported by the Daily Telegraph in London on September 5 1990. But the story lacked the human element; it was an unverified report, there were no pictures for television and no interviews with mothers grieving over dead babies.

That was soon rectified. An organisation calling itself Citizens for a Free Kuwait (financed by the Kuwaiti government in exile) had signed a $10m contract with the giant American public relations company, Hill & Knowlton, to campaign for American military intervention to oust Iraq from Kuwait.

The Human Rights Caucus of the US Congress was meeting in October and Hill & Knowlton arranged for a 15-year-old Kuwaiti girl to tell the babies' story before the congressmen. She did it brilliantly, choking with tears at the right moment, her voice breaking as she struggled to continue. The congressional committee knew her only as "Nayirah" and the television segment of her testimony showed anger and resolution on the faces of the congressmen listening to her. President Bush referred to the story six times in the next five weeks as an example of the evil of Saddam's regime.

In the Senate debate whether to approve military action to force Saddam out of Kuwait, seven senators specifically mentioned the incubator babies atrocity and the final margin in favour of war was just five votes. John R Macarthur's study of propaganda in the war says that the babies atrocity was a definitive moment in the campaign to prepare the American public for the need to go to war.

It was not until nearly two years later that the truth emerged. The story was a fabrication and a myth, and Nayirah, the teenage Kuwaiti girl, coached and rehearsed by Hill & Knowlton for her appearance before the Congressional Committee, was in fact the daughter of the Kuwaiti ambassador to the United States. By the time Macarthur revealed this, the war was won and over and it did not matter any more.

So what should we make of the stories in the British press this week about torture in Afghanistan? A defector from the Taliban's secret police told a reporter in Quetta, Pakistan, that he was commanded to "find new ways of torture so terrible that the screams will frighten crows from their nests". The defector then listed a series of chilling forms of torture that he said he and his fellow officers developed. "Nowhere else in the world has such barbarity and cruelty as Afghanistan."

The story rings false and defectors of all kinds are well-known for telling interviewers what they think they want to hear. On the other hand, it might be true. The trouble is, how can we tell? The media demands that we trust it but too often that trust has been betrayed.

 Phillip Knightley is the author of The First Casualty, a history of war reporting (Prion).