The Chilcot inquiry into the invasion of Iraq – surely one of the defining events of the last decade – may well, if we are lucky, answer some of the pressing questions about that disastrous episode. We may, as a result, be able to confirm with greater certainty that the invasion was illegal, and that it was based on a lie.

What seems unlikely, however, is that we will be any the wiser as to why – from a British standpoint – the invasion was undertaken at all. The question, when applied to the Americans, admits of a relatively straightforward answer. There may have been for George W an element of filial piety, and a sense of a task uncompleted, and controlling the oil may always have been a factor, but the main impetus was surely the conviction of that powerful group of conservatives who controlled the Bush administration – advisers such as Dick Cheney, Donald Rumsfeld, Paul Wolfowitz, Richard Perle – that "if you have the power, use it". The use of what was imagined to be overwhelming American power to change the Middle East map was too tempting to resist.

For these ideologues, alarmingly ignorant as they seem to have been of the world beyond American shores, everyone in the world – whatever their ethnicity, culture or religion – would be Californians if they could; all that was needed was to remove obstacles such as Saddam Hussein, and the dominoes would, for once, fall in the right direction. The invaders, it was confidently predicted, would be welcomed by the liberated with flowers.

But when we ask the question of the British, the answer is less clear, and the Chilcot process seems unlikely to produce any real insights from the only person who could respond with any accuracy and authority. We may be forced to seek the answer for ourselves.

There are of course the explanations derived from realpolitik. There was, first, the supposed need to control future oil supplies and then the constant imperative to stay close to the Americans. The oil question was regularly advanced as a plausible explanation for the Iraq adventure at the time. But, in retrospect, it carries little conviction. There is no evidence that Saddam was any more likely than anyone else to cut off oil supplies to the west; the main obstacle to the continued flow of Iraqi oil was, after all, the sanctions applied by the American-led alliance.

But, it could be argued, if the Americans – even if erroneously – believed that Saddam had to be removed if the oil supply was to be guaranteed, that was surely reason enough to support the invasion, if only to assure the US that it could rely on Britain. And it is certainly true that, following Suez, the imperative to never stray too far from what the Americans wanted was deeply ingrained in British foreign policy, as I discovered at first hand when I joined the Foreign Office in the late 1960s.

Even so, the case for the invasion on the basis that it was essential to do whatever the Americans wanted does not bear scrutiny. If that had been the British attitude, and given the weight of the legal, ethical, military and foreign policy arguments against such a dangerous venture, the sensible course would have been a measured degree of diplomatic support, or at least a defensive refraining from overt opposition. The large-scale and enthusiastic commitment of direct British military support was a step of a wholly different order, and can be explained only by identifying a quite extraordinary additional motivating factor.

That factor was the personality of the then prime minister. It can safely be asserted that, although many could be found at the time to support the invasion, there was no one else in British public life who, given the opportunity, would have had the confidence and moral certainty to take this country to war as Tony Blair did, particularly on the basis of a story that he knew to be false. Where did this amazing chutzpah come from?

Prime ministers who serve a reasonable length of time are always in danger of succumbing to what I call "prime ministerial syndrome" – the belief that, after years of acolytes hanging on their every word, they are infallible. Tony Blair was temperamentally peculiarly susceptible to this condition, exacerbated in his case by his extraordinary ability at that time to persuade the British people of anything he chose. It is easy to see how he came to believe that whether or not the stated reasons for the Iraq invasion were true simply did not matter; the fact that he himself supported the venture was enough.

Why did he support it? He had by this time convinced himself that he was a world statesman, equipped to partner George Bush in a duumvirate which would re-shape the world. Underpinned by a hitherto undeclared religious conviction, he increasingly saw the world in terms of absolutes – good and evil, right and wrong. Like the American conservatives, but for moral and religious reasons rather than misplaced ideological opportunism, he could not resist the chance to strike a blow not only for enlightenment but for his own destiny.

This messianic posture was brilliantly exploited by the Bush administration. After six years of the increasingly tedious and vexatious business of governing Britain, what a wonderful confirmation of his destiny it must have been to receive the unalloyed plaudits of a fawning American establishment and media. The carping of domestic critics could safely be ignored when the world's greatest power recognised him as a saviour. We invaded a foreign country to assure Tony Blair of his place in history. The irony is that it will not be the one he had imagined.