Even former enthusiasts now generally acknowledge that the Anglo-American invasion of Iraq is the greatest disaster in the recent history of US foreign policy. It took almost a decade for the folly of Vietnam to become clear. In Iraq it has taken only a little over a year. Nothing is more important than to try to understand how this catastrophe occurred.

When George Bush came to power there were four genuinely significant influences on the Administration's foreign policy future - Vice-President Dick Cheney; Secretary of State Colin Powell; Defence Secretary Donald Rumsfeld; and his deputy, Paul Wolfowitz, the dominant neo-conservative foreign policy thinker of the age. Bush had hawkish instincts but no policy thoughts.

There are two clear camps. On one side were the neo-cons: Cheney, Rumsfeld and Wolfowitz. On the other the more realist hawk, Colin Powell. The neo-con power base was the Pentagon; the realists' the Department of State. Because in theory they neutralised each other, the Vice-President gained exceptional leverage.

Cheney was a natural extremist and alarmist. In disputes between Rumsfeld and Powell he threw his weight behind the Pentagon. In addition, he used his influence to colonise the Bush Administration with zealous neo-cons. Cheney emerges as the key figure in the Bush Administration. He calls Iraq "Cheney's war".

What were the core elements of the neo-con foreign policy faith?

They believed above all in the need for massive US military spending. Their pet project was missile defence. Colin Powell once joked that after the Cold War he was out of plausible "demons". "I'm down to Castro and Kim Il-sung."

This proved no impediment to the neo-cons. They were interested in such massive military spending because of the conviction that, in what was called "the new American century", the aim of US foreign policy should be to develop such overwhelming military predominance - the preferred euphemism was "strategic depth" - that no other great power or combination of powers would ever again even consider challenging the United States.

The neo-cons also believed in what they called the role of "morality" in foreign affairs. The origin of neo-con interest in morality was their opposition to the post-Vietnam realpolitik of Henry Kissinger and in his sponsorship of the policy of detente with the USSR.

At first for the neo-cons morality was concerned exclusively with anti-communist causes. They despised Jimmy Carter's injection of a principled human rights agenda into international affairs, believing it left communist regimes intact while destabilising pro-American dictators like the shah of Iran.

After the collapse of communism, however, morality became the justification of a more general neo-con program for the application of American ideals - free markets and elections - on a universal scale.

During the 1990s another neo-con obsession was the determination to destroy Saddam Hussein. The neo-cons saw this as the unfinished business of the first Gulf War. They were concerned about the geopolitical influence of Iraq as possessor of 10 per cent of the world's oil reserves. They believed Saddam's removal might aid the strategic interests of Israel or even bring about a Palestinian peace. And they fantasised that if only Iraq could be democratised it might provide a model for the entire Middle East.

During the late 1990s Paul Wolfowitz was the most influential advocate of US military action in Iraq. At this time a fateful alliance was established between the neo-cons and the leader-in-exile of the Iraqi National Congress, Ahmed Chalabi, a convicted criminal whom the neo-cons described as the future George Washington of Iraq.

Following the election of Bush, Wolfowitz made only a little headway with his Iraq plans. With the September 11 attacks on the US, his chances grew.

After September 11 both Rumsfeld and Wolfowitz favoured attacking not Afghanistan but Iraq. On September 16, as Bob Woodward shows, the war question was debated at Camp David. Sniffing the wind, Rumsfeld retreated. Only Wolfowitz now favoured an immediate Iraq war.

Two months later Bush changed his mind. On November 21 he authorised Rumsfeld to prepare a new war plan for Iraq. Rumsfeld worked closely with General Tommy Franks. By late December the outlines of a war plan had been devised. War with Iraq was possible as early as April 2002, although Franks' preferred timetable for invasion was between December 2002 and February 2003. The planning took place in greatest secrecy. As Woodward discovered, as early as March 2002 Franks believed that war with Iraq was almost inevitable.

To go to war a reason was required. As Wolfowitz later explained, for "bureaucratic" purposes the Administration concentrated on Iraq's supposed weapons of mass destruction and links with al-Qaeda. The threat of WMDs and terrorism was far easier to sell to the British and, later, at the United Nations, than "regime change".

One trouble with this argument was that the traditional intelligence agencies were sceptical or uncertain. To compensate for their weakness, the Pentagon created a neo-con intelligence outfit, the Office of Special Plans, whose role was to "cherry-pick" from raw intelligence data and to prepare assessments suitable for use in the Bush Administration's prewar scare campaign.

Until recently the source of this false intelligence was suspected but not known. Last week the Baghdad office of Ahmed Chalabi was raided after discovery of his delivery of secret US intelligence to the security service of Iran. The New York Times now felt obliged to apologise to its readers for the many sensational pre-war WMD reports it had published as a result of Chalabi-inspired help. It was clear that a Western-wide, INC-disseminated disinformation campaign, on Saddam Hussein's weapons and links with al-Qaeda, had provided the neo-cons inside the Bush Administration - unwittingly let us hope - with the casus belli for the war they had long desired. The last piece of the Iraq puzzle now finally fell into place.