It was almost exactly eight years ago that the public beat of the Washington war drums became so loud and insistent that it could no longer be ignored. But we now know that for quite some time before July 2002 Tony Blair and George Bush had been engaged in a dialogue of the determined with regime change in Iraq at the top of their agenda.

Before Chilcot, we had to rely on leaked documents such as telegrams from diplomats, accounts of meetings held round the sofa at No 10, and, for lawyers, the crown jewels of the Attorney General's written advice to the Prime Minister. The Hutton and Butler inquiries helped to fill in some of the blanks, though qualified by their restricted remits and security considerations. But slowly and with only occasional fanfare the whole sad, sorry story is being systematically laid out in evidence before the Chilcot inquiry. Chilcot has not been about surprises but rather about confirmation, less about revelation and more about corroboration of what we thought we knew.

Sir John Chilcot has made it clear that his committee is not a court of law and that no findings of legality will be made but just by exposing to public scrutiny the process by which legal advice was tendered and disregarded, he has provided more than enough evidence in support of the proposition that military action against Iraq was illegal.

This was the central plank of the Liberal Democrat case against war as Nick Clegg reminded the House of Commons on Wednesday. In the aftermath, the attention was upon whether he made that statement as an individual or as a member of the Government. Much more telling was the lack of reaction of the Labour benches in the Commons, including the candidates for leadership, not one of whom will now endorse the decision to go to war in March 2003 which they then supported. Every one a denier, a Simon Peter.

The illegality was then and is today easily stated. Article 2 (4) of the United Nations Charter prohibits regime change. It is hardly surprising that a treaty formed immediately after the Second World War should do so since the Axis powers ignored the sovereignty, territorial integrity and political independence of those whom they sought to annexe or conquer – just as the action against Iraq did.

The Charter of the UN is not the only source of international law. Custom and convention also play a part. It is a principle of customary international law that military action can only ever be justified when all other political and diplomatic alternatives have been exhausted. Until they were forced to leave Iraq by the impending military action, the UN inspectors were still doing their job, as were those of the International Atomic Energy Agency. They were still receiving sufficient co-operation from the Iraqis in the search for WMD. In short, all the diplomatic alternatives had not been exhausted. Military action breached both the UN Charter and customary international law.

What has been most instructive about Chilcot is the fact that all of these arguments were before the Prime Minister, although not, apparently, before the Cabinet! But notwithstanding the legal objections or the predicted consequences for the security of British citizens– as we learned last week from the former head of MI5 Eliza Manningham-Buller – the Prime Minister and those immediately close to him were determined to proceed.

When one considers the public opposition in the street, the opposition of so many in the House of Commons, the illegality of what was being proposed, the risks to life and limb, the threat of terrorism at home and abroad, the potential damage to reputation and the rule of law, and the unknown consequences of toppling Saddam Hussein, two things stand out. First, the unshakeable determination of Tony Blair to press ahead regardless and, second, the unwillingness of Cabinet to assert its authority and rein him in. Were they supine, seduced or submissive? Why did Robin Cook's brave stand not encourage others?

We have, in effect, left Iraq. Its tribulations no longer make the front page. Its current political impasse seems none of our business, although both the ambition of its neighbour Iran, and Turkish resistance to Kurdish aspirations in northern Iraq, should compel us to bear in mind Iraq's regional strategic significance.

So does Iraq matter in Britain? Will Chilcot's conclusions change anyone's views? Will there be concerted cries of mea culpa? Should we lay in stocks of sackcloth and ashes? Hardly, but it does matter because its influence, overt and subliminal, has been profound.

As Baroness Manningham-Buller pointed out in her carefully weighed evidence last week, a generation of young British Muslim men became susceptible to radicalisation (you could argue that it is remarkable so few succumbed) because of their distaste for the Iraq adventure. But the consequences for the security services have been enormous in terms of resources, public exposure and scrutiny.

British forces, and the Army in particular, were put under unexpected and unsustainable pressure. We squandered reputation and resources. We lost too many fine young men and women. But the impact goes wider than that. The reaction against sofa government has produced a commitment to transparency far beyond anything previously seen. Iraq has provoked a debate about the prerogative powers of the Prime Minister, not least in relation to the power to take this country to war. A War Powers Act, akin to that which the Commander-in-Chief and President of the United States has to observe, may yet be the outcome.

Nor are domestic matters only affected. The debate in recent years about our relationship with the US, sharply focused by David Cameron's visit last week, is framed not only by the changed priorities of the Obama administration but also by British reticence to be so close to our principal ally. Iraq casts a long shadow.

In The Independent on Sunday on 16 February 2003, the day after a million people marched through London, I wrote "... most of those who were marching almost certainly understand the deception and ambition of Saddam. What they don't accept is that military action is justified. They are not persuaded that containment and deterrence need to be abandoned. They do not accept that all diplomatic and political alternatives have been exhausted to the point that military action as a last resort is legitimate. They fear the consequences of war in the Middle East and they want justice and a homeland for the Palestinians. In short, public opinion in this country has arrived at a credible foreign policy. This is not anti-Americanism but a lack of confidence in the Bush administration – and a fear that the United Kingdom might end up acting like the 51st state of the union."