They had lost the popular vote, after all, and won the White House only because appointees in the Supreme Court put there by former president George Bush Snr tipped the balance in their favour.

There were three sets of forces present in the new administration that wanted to see Hussein toppled, no matter what: the neo-conservatives, Vice-President Dick Cheney and President George W. Bush. Each had separate but powerful motives, which were there just as much prior to September 11 as after the event.

The neo-conservatives, led by Deputy Defence Secretary Paul Wolfowitz, had, in Mr Bush Snr's administration, managed to push through the labyrinthine Washington process a defining defence policy paper.

It argued for enough American military superiority to stand up to any foreseeable combination of other forces. It also forced through the new idea of pre-emption, something that had never been considered tenable - or acceptable -to the American mind.

Mr Wolfowitz had been defence undersecretary in the previous Bush administration, and became known as the progenitor of the new doctrine. Few people thought of the likely application of its cluster of ideas - until, in fact, 2001, when he and his neo-conservative allies were ensconced throughout the administration.

Early in the present Bush administration, however, it became clear that a trio of neo-conservatives at the Pentagon had highly specific designs on policy and even geography: Mr Wolfowitz, the new undersecretary Douglas Feith, and the old warrior of hardline defence politics, Richard Perle - known as the Prince of Darkness during the Reagan years.

As American Jews with close ties to Israel, they sought to use any excuse to redraw the map of the Middle East in Israel's favour, and this they saw - without a second thought to its implications - as a decided American interest. They saw Israel as a firm American ally, a democracy in a sea of anarchy. That such a policy would alienate their country from one billion Muslims was irrelevant. That such a policy might seem self-serving, even in some quarters leading to the question of dual-loyalties, they also considered manageable. The gains were just too overwhelming. They just could not resist the temptation.

Why hit Iraq? Hussein had targeted Israel in the earlier Gulf war. After that, Hussein gave a huge bounty to the family of every Palestinian suicide bomber, one of the most profound incentives to potential young martyrs.

More importantly, in the mind of the neo-conservatives, Hussein could readily be destroyed - and once toppled, the tanks that had done the work could roll on, as there were six more target countries, all Islamic enemies of Israel.

Mr Wolfowitz gave the game away in a celebrated interview with Vanity Fair magazine in which he observed that weapons of mass destruction was agreed on as the excuse for the war against Hussein as it was the most readily marketable.

Mr Cheney is the self-defined tough guy, the king (or king-maker) in the middle, around whom all other players revolve. Washington insiders all knew that he meant to topple Hussein from the start. Why? He wanted to show that America could flex its muscles anywhere, hit the target and move on. From the start, he dismissed all arguments against a war to break Hussein as weak. He backed all those, whatever their motives, who were rallying to topple the dictator.

And then there is the president himself. We know how little he knew about foreign affairs; no matter. He knows about family - as anyone whose father was president, whose brother was a governor, and about whose own capabilities there were grave doubts - would know. He knew two things about Hussein: he had tried to kill his father - giving a loyal son an overwhelming and acceptable motive for revenge - and that Hussein was still in power, despite his father's war in 1991, which had only forced the dictator to disgorge Kuwait.

Mr O'Neill may not have grasped the inner subtleties of the motives. But he knew a war plan when he heard it. All that was lacking was a pretext - and when the Twin Towers came down, the war plan went up.

Scott Thompson was an assistant to the US secretary of defence from 1975 to 1976, and served as an assistant secretary of state in the Reagan administration. He is a professor at the Fletcher School of Law and Diplomacy in Massachusetts