Brian Jones: I'm still mystified how Blair convinced himself that Iraq had all those weapons

The headline conclusion of the Iraq Survey Group (ISG) report-- that Iraq had no significant stockpiles of chemical or biological weapons - came as little surprise to me. The assessments that my colleagues and I on the Defence Intelligence Staff made in 2002 suggested that this might be the case, a view that was rejected by the Prime Minister, his team at No 10, the Cabinet, the Cabinet Office and the Joint Intelligence Committee (JIC).

I am, however, surprised at the finding of the ISG that the state of Saddam Hussein's WMD programmes and the related knowledge base was weaker in 2003 than in 1998 when United Nations weapons inspectors and monitors were ordered by Saddam to leave Iraq. Although some of the intelligence we received after 1998 hinted as much, it was difficult to square these reports with others, and with the continuing failure of Iraq to cooperate fully with the inspectors. If there was nothing tangible to be found, why was Iraq contriving to give the impression that there might be? What now seems to have been the case is that Saddam felt he needed a strategic deterrent, and for as long as he did not have one, the next best thing was to create the illusion that he did.

What remains a mystery is how most senior members of the Blair Government and most senior British intelligence officials convinced themselves of the existence of stockpiles of Iraqi chemical and biological weapons when their expert analysts in the middle ranks were so sceptical.

Perhaps what really happened is that they shared our assessment of the risk that Iraq might eventually reconstitute its WMD capabilities to the point where it could deter us from intervention. That would infringe Britain's declared policy aim of maintaining our political and military freedom of action. So they decided to grasp the emerging opportunity to deal with it.

But chemical, biological and nuclear weapons are each as complex as they are different. Explaining to busy MPs and a barely engaged public the nuanced implications of a range of emerging potential threats including unconstrained global terrorism would have been a challenge even in a more relaxed environment. Under the pressure generated by the haste of Bush administration hawks to take advantage of America's rage at the 11 September attacks, it would have seemed impossible. So whether out of ignorance or pragmatism, the message was simplified, and prompted stark headlines ("Brits 45 minutes from doom") that went unchallenged.

There were other benefits in this simplification when it came to making a case for war. All discussion could be persuaded to default to the worst case scenario - the mushroom clouds that presaged Hiroshima's desolate aftermath. Of course, it was never assessed by anyone that Iraq had one, let alone a stockpile of nuclear weapons. And the trumpeted evidence of deals sought in the uranium mines of Niger, and aluminium tubes for its enrichment detected in transit to Iraq, were all so much froth.

Although the assessment of the Defence Intelligence Staff about the lack of any stockpiles was not completely accurate either, it was much closer to what Charles Duelfer's Iraq Survey Group has discovered. Indeed, it is pleasing to realise that our assessment was probably more accurate than the belief held by some of Saddam's most senior Iraqi military officers, whom the report tells us were convinced of the existence of WMD until the dictator told them otherwise in December 2002.

Given that even Saddam's most senior officers were misled, it is perhaps easier to see how inaccurate intelligence reports about stockpiles came through, and that some of these were subsequently given too much credibility in the September 2002 dossier. But even without the knowledge of the deception going on inside the corridors of power in Baghdad, we were suspicious about these reports because there was simply no supporting evidence. This is not rocket science, just sound intelligence analysis.

The requirements of selling the war, however, demanded a strong case. And that case would have been difficult to sustain if based merely on a weapons capability kept alive in the memories of Iraqi heads and hard drives, and held in reserve for activation on some undefined future date.

In the light of his adamant claims about the existence of significant actual WMD stockpiles then, the Prime Minister's perfectly valid claims now about the threats associated with Saddam's intentions and incipient potential capabilities ring hollow. For still, he cannot bring himself to use the important qualifier "potential". He reiterates that a mass of starkly clear intelligence misled him into believing the threat was actual, here and now, and yet seems remarkably unconcerned that an estimated £1.5bn-plus per annum intelligence machine could get it so wrong on such a vital issue that has cost the nation so much.

But I don't recall that Lord Butler's review uncovered masses of clear-cut evidence indicating the existence of a current WMD threat to anyone, let alone Britain.

The tragedy is that the Prime Minister is right about "terrorism without limit". Yet the risk of it being visited upon our shores has been greatly enhanced by the very decision he believes he took to protect us from it. And now our ability to defend against it is being hampered by his refusal to acknowledge fully the mistakes that he and others made in relation to the Iraq war.

Tony Blair told John Humphrys last week that, post 11 September, he decided we had to take a proactive approach to WMD, and the place to start was with Iraq. If Iraq is just the start, then it is his urgent responsibility to explain the overall strategy in much greater detail.

There are two important aspects of the problem that are being glossed over. The first is how to reorganise the machinery of government to avoid the periodic corruption of intelligence by policy. It seems to recur every decade or so, and requires a more enduring resolution than a quick fix.

The second concerns our policy on countering WMD proliferation, especially now as arms control crumbles and global terrorists are intent on producing mass casualties. Aggressive attempts to suppress the legitimate desire of nations to protect themselves from neighbours armed with weapons of mass destruction will not succeed. It is difficult to see that even a future democratic Iraqi regime would not try to equip itself with a strategic deterrent unless we undertake to guarantee its security. The solutions must be pursued through an international consensus that offers reassurance and security to those we want to renounce such weapons. But the ultimate rejection by the British Government, with the full support of the Opposition, of the consensus view that existed in the United Nations on Iraq has undoubtedly made this more difficult.

The writer was a member of the Defence Intelligence Staff until January 2003, and is a visiting senior research fellow at University of Southampton.