It will be a gross injustice if the intelligence services get the blame

Britain joined the war because the Prime Minister wanted to prove to President Bush that Tony Blair was his best friend

Trying to pin down why the Government thinks it went to war is proving almost as difficult as getting reliable intelligence of Iraq's weapons capacity under Saddam. Until this weekend much of Britain had been misled into believing that we invaded Iraq because ministers had spine-chilling evidence that Saddam had acquired weapons of mass destruction with which to zap us, if we did not hit him first. Then yesterday the Prime Minister appeared before Liaison Committee to brush aside with impatience the notion that the case for war turned on anything as tangible as a stockpile of weapons hardware.

It may help to jog the collective memory of ministers if we go back to the texts they themselves wrote at the time. The motion they put before Parliament on the eve of war asked the Commons to "support the decision of the Government that the UK should use all means necessary to ensure the disarmament of Iraq's weapons of mass destruction".

The other argument for war deployed at the time was that invasion was necessary as part of the war against terrorism and to prevent the fabled weapons of mass destruction falling into the hands of al-Qa'ida. It is probably as well for us all that the weapons of mass destruction did not turn out to exist because Iraq is now awash with international terrorists as a result of our invasion and the consequent collapse of border security.

Meanwhile the war and the subsequent occupation is proving a spectacular own goal in the war on terrorism. On Monday the Foreign Affairs Select Committee reported that the failure to find weapons of mass destruction "has damaged the credibility of the US and the UK in their conduct of the war against terrorism" and that the war "has possibly made terrorist attacks against British nationals and British interests more likely in the short term".

In short the war has neither disarmed a single weapon of mass destruction nor diminished the terrorist threat to British interests. It has, though, undermined the authority of the UN, divided us from our major partners in Europe and damaged our status in the Third World, especially Muslim countries. The Iraq war is proving the greatest blunder in British foreign and security policy since Suez.

And in response to the swelling mountain of evidence that the Government got it wrong, we were yesterday offered a mouse of an inquiry. As Number 10 has drawn parallels with the Franks committee into the Falklands war, it is instructive to contrast the broad remit of that inquiry with the constricted terms of reference of the new Butler inquiry.

Franks was set up to review "the responsibilities of Government" in the run-up to the Falklands war. There is no hint anywhere in the remit of the new Butler inquiry that ministers had any possible responsibility for what went wrong, and most certainly no suggestion that its distinguished membership should waste their time contemplating such an old-fashioned constitutional principle as ministerial responsibility.

Instead they are to confine themselves to the narrow question of the accuracy of intelligence, as if the intelligence judgement led us to go to war on the basis of that threat. The reality is that intelligence from Iraq was rifled in order to find the scraps to support a political decision.

There has been a tendency in recent weeks to discuss intelligence as if it is proven scientific fact. It is not. If the information could be obtained from public, open sources we would not need a secret service to find it. The job of intelligence agencies is to peer into dark corners of the globe and try to imagine a whole jigsaw with a dozen pieces they have gleaned from human gossip and radio intercepts. Every intelligence assessment I ever read was meticulously careful to qualify the reliability of the data and to acknowledge there could be alternative interpretations.

It would be a gross injustice if the intelligence agencies were now to carry the can for a war built on such slender foundations. At the very least the Butler inquiry needs to ask what the intelligence agencies were tasked to find out on Iraq and whether Number 10 ever asked a sceptical question about the intelligence before it so readily pushed it in public.

But there is a greater falsity in confining the terms of reference to intelligence. The truth is that Tony Blair did not take Britain into Iraq because of any evidence of weapons of mass destruction. He joined in the war because he wanted to prove to President Bush that Tony Blair was his best friend and Britain was his most reliable ally. The Butler inquiry is a diversion, set up to examine the pretext for war rather than its origins.

To get at the real reasons why Britain went to war, we would need to know what passed between Number 10 and the White House over the preceding year. If it turns out that the White House had reason to believe Britain would take part in an invasion of Iraq even before publication of the September dossier, there will be little point in worrying about why its claims have turned out to be so wildly wrong and even less point trying to blame the intelligence agencies for the decision to go to war.

Lord Butler will get short shrift from Number 10 if he has the impertinence to stray that far beyond his official brief. Yet we may find out anyway as a result of the parallel inquiries in the States. The final confirmation of our junior status in the special relationship was the decision that Britain should have an inquiry solely because President Bush had already decided to hold one for the US. Nothing could more pointedly demonstrate how we have become humiliatingly dependent on the initiatives taken in the Oval Office.

We will though have to wait longer on the outcome as the White House is briefing that next year is the target date for the report of the US inquiry. We are all grown-ups and we all know the real reason for that leisurely pace. It suits President Bush just fine to have the inquiry kicked into the blue yonder on the far side of the Presidential elections. For Tony Blair it is a potential disaster. The consequence could be an embarrassing report on the war coming out on the eve of the next General Election.

I do not know if President Bush was aware of that awkward piece of political timing. What is obvious is that even if he knew that this could be difficult for his friend in Downing Street he did not give it a second thought. At one of my last meetings with Tony Blair before I resigned I warned him that many in the White House would regard it as a bonus if controversy over Iraq weakened a leftist administration in Britain. I hope after the past week that the lesson has not been lost on him that, despite the political gamble he has taken on behalf of President Bush, there is no willingness over there to protect him from a political headache if it gets in the way of easing their own domestic pressures.