Iraq invaded Kuwait on August 2 1990. The US cabinet met the same day. At that point, war was no more than a possibility. Norman Schwarzkopf recalls the prevailing mood in his autobiography, It Doesn't Take a Hero. He quotes General Colin Powell's remark to him: "I think we could go to war if they invaded Saudi Arabia. I doubt if we would go to war over Kuwait."

Within days the mood at the top had hardened. When Schwarzkopf next met Powell, he was told to prepare to go to Saudi Arabia. "I was stunned," he says in his book. "A lot must have happened after I left Camp David that Powell wasn't talking about. President Bush had made up his mind to send troops."

A lot had changed. By the early weeks of September, America and Britain were leading the march towards war. Somehow, almost without anybody noticing, the agenda was changing. Iraqi withdrawal from Kuwait alone was no longer acceptable. New resolutions had been adopted by the UN security council

The photographs, which are still classified in the US (for security reasons, according to Brent Scowcroft, President Bush senior's national security advisor), purportedly showed more than a quarter of a million Iraqi troops massed on the Saudi border poised to pounce. Except, when a resourceful Florida-based reporter at the St Petersburg Times persuaded her newspaper to buy the same independently commissioned satellite photos from a commercial satellite to verify the Pentagon's line, she saw no sign of a quarter of a million troops or their tanks.

Jean Heller, an investigative reporter on the St Petersburg Times, has been nominated for a Pulitzer prize five times and come second twice, so when she asked permission to spend $3,200 (£1,950) on two satellite pictures, the newspaper backed her.

Heller's curiosity had been aroused in September when she read a report of a commercial satellite - the Soyuz Karta - orbiting and taking pictures over Kuwait. She wanted to see what the only independent pictures would make of the alleged massive build-up of Iraqi troops on the Kuwait/Saudi border. Soyuz Karta agreed to provide them. But no trace of the 265,000 Iraqi troops and 1,500 tanks that the US officials said were there could be found in the photographs.

"The satellite pictures were so clear that at Riyadh airport in Saudi Arabia you could see American planes sitting wingtip to wingtip," Heller says.

She took the photographs for analysis to two experts. "I looked at them with a colleague of mine and we both said exactly the same thing at exactly the same moment: 'Where are they?'" recalls Peter Zimmerman, a satellite expert at George Washington University.

'We could see clearly the main road leading right through Kuwait, south to Saudi Arabia, but it was covered with sand banks from the wind and it was clear that no army had moved over it. We could see empty barracks where you would have expected these thousands of troops to be billeted, but they were deserted as well."

Jean Heller wrote her story for the St Petersburg Times. It opened with the words: "It's time to draft Agatha Christie for duty in the Middle East. Call it, The Case of the Vanishing Enemy."

Looking back now, Heller says: "If the story had appeared in the New York Times or the Washington Post, all hell would have broken loose. But here we are, a newspaper in Florida, the retirement capital of the world, and what are we supposed to know?"

A year later, Powell would admit to getting the numbers wrong. There was no massive build-up. But by then, the war had been fought.

A public relations firm on a $2m contract from the Kuwaiti government had been surreptitiously employed to make the case for war. Hill & Knowlton's coup de grace was their fabricated "incubator baby" story. A story of how Iraqi soldiers had thrown premature babies out of incubators in the Al Adnan hospital in Kuwait city and "left them on the cold floor to die".

Hill & Knowlton's work involved coaching six witnesses to give the fake details of the attack on the premature baby unit. The story was graphically told to Congress in November 1990 - before a crucial vote - by Niyirah al Sabah who, unknown to her audience, was the daughter of the Kuwaiti ambassador to the US. In her tearful testimony, she said she had witnessed the Iraqi troops' brutality when she worked as a volunteer in the maternity ward.

But Myra Ancog-Cooke, a Filipino nurse who worked in the hospital, said that none of the staff there had ever heard of Niyirah al Sabah; they had been present in the hospital throughout the Iraqi occupation of Kuwait and the story was untrue. A staunch Catholic, Ms Ancog-Cooke explained that it was her duty and God's will that she stayed to care for the sick. She was assigned to the children's ward and took it in turns with the other Filipino nurse who stayed behind, Freida Contrais-Naig, to sleep in the incubator room with the babies.

"I remember someone called and said, 'Look at CNN, they are talking about us." We watched and it was strange seeing that girl telling them about the Iraqis taking the babies out of the incubators. I said to Freida, 'That's funny, we've never seen her. She never worked here.' We didn't think very much about it really. We were more excited seeing our hospital on the television," she says.

Later, Amnesty International, who had also been duped by the testimony, admitted it had got it wrong. Andrew Whitley of Middle East Watch described it as a fabrication, but it took months for the truth to come out. Meanwhile, President Bush mentioned the incubator babies in five speeches and seven senators referred to them in speeches backing a pro-war resolution.