Did US troops use chemical weapons in Falluja? The answer is yes. The proof is not to be found in the documentary broadcast on Italian TV last week, which has generated gigabytes of hype on the internet. It's a turkey, whose evidence that white phosphorus was fired at Iraqi troops is flimsy and circumstantial. But the bloggers debating it found the smoking gun.

The first account they unearthed in a magazine published by the US army. In the March 2005 edition of Field Artillery, officers from the 2nd Infantry's fire support element boast about their role in the attack on Falluja in November last year: "White Phosphorous. WP proved to be an effective and versatile munition. We used it for screening missions at two breeches and, later in the fight, as a potent psychological weapon against the insurgents in trench lines and spider holes when we could not get effects on them with HE [high explosive]. We fired 'shake and bake' missions at the insurgents, using WP to flush them out and HE to take them out."

The second, in California's North County Times, was by a reporter embedded with the marines in the April 2004 siege of Falluja. "'Gun up!' Millikin yelled ... grabbing a white phosphorus round from a nearby ammo can and holding it over the tube. 'Fire!' Bogert yelled, as Millikin dropped it. The boom kicked dust around the pit as they ran through the drill again and again, sending a mixture of burning white phosphorus and high explosives they call 'shake'n'bake' into... buildings where insurgents have been spotted all week."

White phosphorus is not listed in the schedules of the Chemical Weapons Convention. It can be legally used as a flare to illuminate the battlefield, or to produce smoke to hide troop movements from the enemy. Like other unlisted substances, it may be deployed for "Military purposes... not dependent on the use of the toxic properties of chemicals as a method of warfare". But it becomes a chemical weapon as soon as it is used directly against people. A chemical weapon can be "any chemical which through its chemical action on life processes can cause death, temporary incapacitation or permanent harm".

White phosphorus is fat-soluble and burns spontaneously on contact with the air. According to globalsecurity.org: "The burns usually are multiple, deep, and variable in size. The solid in the eye produces severe injury. The particles continue to burn unless deprived of atmospheric oxygen... If service members are hit by pieces of white phosphorus, it could burn right down to the bone." As it oxidises, it produces smoke composed of phosphorus pentoxide. According to the standard US industrial safety sheet, the smoke "releases heat on contact with moisture and will burn mucous surfaces... Contact... can cause severe eye burns and permanent damage."

Until last week, the US state department maintained that US forces used white phosphorus shells "very sparingly in Fallujah, for illumination purposes". They were fired "to illuminate enemy positions at night, not at enemy fighters". Confronted with the new evidence, on Thursday it changed its position. "We have learned that some of the information we were provided ... is incorrect. White phosphorous shells, which produce smoke, were used in Fallujah not for illumination but for screening purposes, ie obscuring troop movements and, according to... Field Artillery magazine, 'as a potent psychological weapon against the insurgents in trench lines and spider holes...' The article states that US forces used white phosphorus rounds to flush out enemy fighters so that they could then be killed with high explosive rounds." The US government, in other words, appears to admit that white phosphorus was used in Falluja as a chemical weapon.

The invaders have been forced into a similar climbdown over the use of napalm in Iraq. In December 2004, the Labour MP Alice Mahon asked the British armed forces minister Adam Ingram "whether napalm or a similar substance has been used by the coalition in Iraq (a) during and (b) since the war". "No napalm," the minister replied, "has been used by coalition forces in Iraq either during the war-fighting phase or since."

This seemed odd to those who had been paying attention. There were widespread reports that in March 2003 US marines had dropped incendiary bombs around the bridges over the Tigris and the Saddam Canal on the way to Baghdad. The commander of Marine Air Group 11 admitted that "We napalmed both those approaches". Embedded journalists reported that napalm was dropped at Safwan Hill on the border with Kuwait. In August 2003 the Pentagon confirmed that the marines had dropped "mark 77 firebombs". Though the substance these contained was not napalm, its function, the Pentagon's information sheet said, was "remarkably similar". While napalm is made from petrol and polystyrene, the gel in the mark 77 is made from kerosene and polystyrene. I doubt it makes much difference to the people it lands on.

So in January this year, the MP Harry Cohen refined Mahon's question. He asked "whether mark 77 firebombs have been used by coalition forces". The US, the minister replied, has "confirmed to us that they have not used mark 77 firebombs, which are essentially napalm canisters, in Iraq at any time". The US government had lied to him. Mr Ingram had to retract his statements in a private letter to the MPs in June.

We were told that the war with Iraq was necessary for two reasons. Saddam Hussein possessed biological and chemical weapons and might one day use them against another nation. And the Iraqi people needed to be liberated from his oppressive regime, which had, among its other crimes, used chemical weapons to kill them. Tony Blair, Colin Powell, William Shawcross, David Aaronovitch, Nick Cohen, Ann Clwyd and many others referred, in making their case, to Saddam's gassing of the Kurds in Halabja in 1988. They accused those who opposed the war of caring nothing for the welfare of the Iraqis.

Given that they care so much, why has none of these hawks spoken out against the use of unconventional weapons by coalition forces? Ann Clwyd, the Labour MP who turned from peace campaigner to chief apologist for an illegal war, is, as far as I can discover, the only one of these armchair warriors to engage with the issue. In May this year, she wrote to the Guardian to assure us that reports that a "modern form of napalm" has been used by US forces "are completely without foundation. Coalition forces have not used napalm - either during operations in Falluja, or at any other time". How did she know? The foreign office minister told her. Before the invasion, Clwyd travelled through Iraq to investigate Saddam's crimes against his people. She told the Commons that what she found moved her to tears. After the invasion, she took the minister's word at face value, when a 30-second search on the internet could have told her it was bunkum. It makes you wonder whether she really gave a damn about the people for whom she claimed to be campaigning.

Saddam, facing a possible death sentence, is accused of mass murder, torture, false imprisonment and the use of chemical weapons. He is certainly guilty on all counts. So, it now seems, are those who overthrew him.

Monbiot.com