Page by relentless page, evidence has been stacking up for many months to show that - despite Tony Blair's denials - the British government signed up for war in Iraq almost a year before the invasion. What most people will not have realised until now, however, was that Britain and the US waged a secret war against Iraq for months before the tanks rolled over the border in March 2003. Documentary evidence and ministerial answers in parliament reveal the existence of a clandestine bombing campaign designed largely to provoke Iraq into taking action that could be used to justify the start of the war.
In the absence of solid legal grounds for war, in other words, the allies tried to bomb Saddam Hussein into providing their casus belli. And when that didn't work they just stepped up the bombing rate, in effect starting the conflict without telling anyone.
The main evidence lies in leaked documents relating to a crucial meeting chaired by the Prime Minister in July 2002 - the documents which supported the Sunday Times story, published during this past election campaign, about how Blair promised George W Bush in April that year that Britain would back regime change.
A briefing paper for the ministers and officials at the meeting - this was in effect a British war cabinet - laid out two alternative US war plans. The first, a "generated start", involved a slow build-up of roughly 250,000 troops in Kuwait. Allied aircraft would then mount an air war, which would be followed by a full-scale invasion. The second option was a "running start", in which a continuous air campaign, "initiated by an Iraqi casus belli", would be mounted without any overt military build-up. Allied special forces giving support to Iraqi opposition groups on the ground would be joined by further troops as and when they arrived in theatre, until the regime collapsed. A few days after the meeting, the Americans opted for a hybrid of the two in which the air war would begin, as for a running start, as soon as the Iraqis provided the justification for war, while at the same time an invasion force would be built up, as for a generated start.
The record of the July meeting in London, however, contains a revealing passage in which Geoff Hoon, then defence secretary, tells his colleagues in plain terms that "the US had already begun 'spikes of activity' to put pressure on the regime". What is meant by "spikes of activity" becomes clear in the light of information elicited from the government by the Liberal Democrat Sir Menzies Campbell, who asked the Ministry of Defence about British and American air activity in 2002 in the southern no-fly zone of Iraq - the zone created to protect southern Shias after Saddam Hussein brutally suppressed their 1991 uprising against him.
The MoD response shows that in March 2002 no bombs were dropped, and in April only 0.3 tonnes of ordnance used. The figure rose to 7.3 tonnes in May, however, then to 10.4 in June, dipping to 9.5 in July before rising again to 14.1 in August. Suddenly, in other words, US and British air forces were in action over Iraq.
What was going on? There were very strict rules of engagement in the no-fly zones. The allied pilots were authorised to fire missiles at any Iraqi air defence weapon or radar that fired at them or locked on to their aircraft. As was noted in Foreign Office legal advice appended to the July 2002 briefing paper, they were only "entitled to use force in self-defence where such a use of force is a necessary and proportionate response to actual or imminent attack from Iraqi ground systems".
That May, however, Donald Rumsfeld had ordered a more aggressive approach, authorising allied aircraft to attack Iraqi command and control centres as well as actual air defences. The US defence secretary later said this was simply to prevent the Iraqis attacking allied aircraft, but Hoon's remark gives the game away. In reality, as he explained, the "spikes of activity" were designed "to put pressure on the regime".
What happened next was dramatic. In September, the amount of ordnance used in the southern no-fly zone increased sharply to 54.6 tonnes. It declined in October to 17.7 tonnes before rising again to 33.6 tonnes in November and 53.2 tonnes in December. The spikes were getting taller and taller.
In fact, as it became clear that Saddam Hussein would not provide them with the justification they needed to launch the air war, we can see that the allies simply launched it anyway, beneath the cloak of the no-fly zone.
In the early hours of 5 September, for example, more than a hundred allied aircraft attacked the H-3 airfield, the main air defence site in western Iraq. Located at the furthest extreme of the southern no-fly zone, far away from the areas that needed to be patrolled to prevent attacks on the Shias, it was destroyed not because it was a threat to the patrols, but to allow allied special forces operating from Jordan to enter Iraq undetected.
It would be another nine weeks before Blair and Bush went to the UN to try to persuade it to authorise military action, but the air war had begun anyway. The number of raids shot up, from four a month to 30, with allied aircraft repeatedly returning to sites they had already hit to finish them off. Senior British officials insist that no RAF aircraft opened fire until it was at least locked on to by an Iraqi radar, but it is difficult to see how the systematic targeting of Iraqi installations could have constituted "a necessary and proportionate response". The story of the secret air war dovetails neatly with the other evidence from the leaked documents, further demonstrating why, even after the general election, Blair's efforts to dispel the allegations about the background to war and get the country to "move on" seem doomed to fail.
It was the briefing paper for the July meeting which stated categorically that "when the Prime Minister discussed Iraq with President Bush at Crawford in April , he said that the UK would support military action to bring about regime change".
The same document also stated bluntly that "regime change per se is not a proper basis for military action under international law" and it was therefore "necessary to create the conditions in which we could legally support military action".
America had none of these problems. It was Washington's view that it could decide for itself whether Saddam was in breach of his obligations to let in weapons inspectors. With British officials holding Blair back, insisting that without UN backing an invasion would be illegal, it would have been extremely convenient for Bush and Rumsfeld if Saddam had retaliated against the bombing offensive, thus giving London and Washington the chance to cry, "He started it!"
The leaked British documents have now found their way into the US political debate. The White House has declined to respond to a letter from 89 US congressmen asking Bush when he and Blair agreed to invade Iraq. The congressmen are now talking about sending a delegation to Britain to try to find out the truth, although heaven alone knows why they think they will get any more change from Blair than they did from Bush. Their concerns are none the less grave ones, for the leaked documents are as damaging to Bush as they are to Blair.
Under the US constitution, only Congress has the power to authorise war, and it did not do so until 11 October. Any military ac-tion to oust Saddam before that point would constitute a serious abuse of power by the president. But there is no reason to suppose that bothered Mr Bush.
Michael Smith writes on defence matters for the Sunday Times