Denunciations of Tony Blair as the evil architect of Britain’s involvement in the Iraq War often dominate discussions of what happened there and many will look to the Chilcot inquiry to provide further evidence of his guilt. But the demonisation of Mr Blair is excessive and simple-minded and diverts attention from what really happened in Iraq and how such mistakes can be avoided in future.

He may have unwisely followed the US into the quagmire of Iraq, but British government policy since 1941 has been to position itself as America’s most loyal and effective ally in peace and war.

There have been significant exceptions to this rule, such as the Suez Crisis and the Vietnam War, but during the last 70 years the UK has generally sought to influence US policy in its formulation and then support it unequivocally once adopted.

The Chilcot Inquiry - A timeline of the Iraq War

This may on occasions be humiliating and out of keeping with the British self-image of robust independence, but it is not as stupid from the point of view of the British state.

In going to war in alliance with the US in Iraq, Mr Blair was not doing anything very different from his predecessors or successors, except that he was more successful than them in establishing close relations with the White House.

Many British politicians, diplomats, academics and journalists have subsequently claimed they were convinced at the time that the invasion would end badly, but they were notably quiet about their forebodings at the time and for most part their wisdom is retrospective. Nor that British opposition to the US venture in Iraq could have done much to stop it happening.

I was in Washington in the first months of 2003, working for a think tank and it was clear that the invasion was going to go ahead whatever London said or did. In the hysterical super-patriotic atmosphere post 9/11, it was difficult for Americans to oppose the war and those who told me that they had doubts about it – one academic expert said he was sure it would be as disastrous for the US as the Suez Crisis had been for Britain – were careful not to say so publicly.

The Bush administration had a lethal combination of hubris, ideological preconceptions and ignorance, which made it impossible to warn them about the multitude of dangers they would face in Iraq; and even if they had been more open-minded, they understandably felt that they had heard similar scepticism prior to the Afghan war in 2001, yet they and their local allies had apparently defeated the Taliban easily and at little cost. The temporary nature of this victory was not yet apparent.

The biggest and most damaging failing of the Americans and the British during the Iraq War, which eventually doomed the whole enterprise, was that they both underrated the importance of how Iraqis and states neighbouring Iraq would respond to the invasion and occupation.

Shortly before I left Washington in early 2003, a senior American journalist spelled out to me US plans for ruling Iraq post-war and I said that I did not believe that these would go down well with Iraqis. “Who cares what Iraqis think?” he replied. “Who cares?” This was the pervasive attitude among those running the war and later during the first disastrous months of the occupation, an overconfidence encouraged by the swift collapse of the Iraqi army which appeared to show that Iraqis were not prepared to resist the foreign presence (in reality, they were not willing to fight for Saddam Hussein).

The occupiers forgot the old nostrum that “the enemy also gets a vote [in determining what happens]”. To this day, British officials engage in retrospective wishful thinking, imagining that all would have been well if the US and Britain had a better plan for ruling Iraq post-invasion or enough troops to guard its frontiers against foreign infiltration, but neither of these would have made much difference in the long term.

The invasion and occupation of Iraq are often mistakenly treated as one event, when in fact they were separate. The invasion might have succeeded if it had been a brief foray limited to getting rid of Saddam Hussein who most Iraqis by this time either hated or regarded as a disaster for his country.

But instead the invasion was followed by occupation in which all power was seized by the invading powers who ruled Iraq directly or through local proxies. They did this because they did not want the political vacuum created by the fall of Saddam Hussein to be filled by Iran or the Shia religious parties in Iraq, but did not see that an occupation was unacceptable to all Iraqis – though the reasons were different for Sunni and Shia – aside from the Kurds who were not occupied.

Soldiers from the The Royal Welsh Fusiliers mount helicopter check points around Basra in 2004 (Getty)

The US and Britain also underestimated the extent to which the overthrow of Saddam inevitably meant a political, economic and social revolution as the majority Shia population took over from the Sunni as the dominant community.

The occupiers soon found that they were presiding over or participating in a sectarian war of extraordinary savagery. Making this situation even more lethal for the Americans and British was that none of Iraq’s neighbours wanted a Western land army permanently on their doorsteps. Iranian and Syrian leaders heard hawks in the US boast that regime change in Baghdad was only an appetiser for the same thing in Damascus and Tehran, thereby guaranteeing that Syria and Iran would support anybody prepared to fight and keep the US busy in Iraq.

What should Britain have done in these circumstances? What unforced errors did it make? Could they have been avoided and were there alternative policies available? Britain may have traditionally supported the US, but it was unwise to give blank cheques to an administration in Washington so prone to exaggerate its own strength and underestimate that of others. It was inevitable that the invasion would get rid of Saddam, but it should have been foreseeable that the American and British commitment would be open-ended because regime-change would inevitably open the door to sectarian and regional conflicts that would permanently destabilise Iraq.

The Iraq War in Numbers

Such a long war could not be fought successfully with public opinion so divided at home. Even in retrospect, it was astonishing how little the British and Americans felt it necessary to know about the country they were going to invade and occupy.

The Justice Secretary and candidate for leadership of the Conservative Party, Michael Gove, was much criticised during the Brexit campaign for pouring scorn on the value of experts, but he was only repeating an attitude fostered for almost 20 years under Mr Blair and Mr Cameron.

The opposite of the expert is the amateur and, for all the skills as communicators of the two prime ministers, there was always a strong smell of amateurism about their reaction to events in Iraq, Libya and Syria, not to mention the other wars and insurgencies exploding across the Middle East and North Africa.

It was an approach that hobbled Britain’s military and diplomatic effectiveness. The Foreign Office no longer saw expertise in and experience of particular countries as an essential for a diplomat intent on a successful career.

How the British media covered the Iraq War Show all 6 1 /6 How the British media covered the Iraq War How the British media covered the Iraq War Daily Mirror, 6 January 2003 How the British media covered the Iraq War The Guardian, 21 March 2003 How the British media covered the Iraq War The Daily Mail, 22 March 2003 How the British media covered the Iraq War Daily Telegraph, 21 March 2003 How the British media covered the Iraq War The Sun, 20 March 2003 How the British media covered the Iraq War

One former diplomat who speaks Arabic and Farsi says that his promotion was delayed because he was deemed over-specialised. British Army generals in Iraq and Afghanistan were moved in and out of the war zones so quickly that they could not get a real grip on what was happening on the ground. British ministers usually appeared ill-informed about the main features of the political and military landscape in which they were sending soldiers to fight.

Experience of war should have been a swift if deadly teacher in Iraq and the US Army did come to understand the human terrain in which it was operating. But the British seemed inhibited from learning from their mistakes on the ground by pretending that they had not happened. Investigations focused narrowly on the misuse of intelligence in justifying the war.

The British Army and the Americans were visibly unprepared for the menu of tactics used by the Iraqi nationalist movements and al-Qaeda clones which included the mass use of suicide bombers and Improvised Explosive Devices (IEDS) mixed in with more regular infantry weapons.

The key players in the Iraq War Show all 11 1 /11 The key players in the Iraq War The key players in the Iraq War Jack Straw Jack Straw was the UK foreign secretary at the time of the Iraq invasion, and fully endorsed the decision Getty The key players in the Iraq War Geoff Hoon Geoff Hoon was Tony Blair’s defence secretary from October 1999 to May 2005 Getty Images The key players in the Iraq War Alastair Campbell Alastair Campbell was involved in the drafting of two Downing Street dossiers on the war, in September 2002 and in February 2003 Getty The key players in the Iraq War John Scarlett John Scarlett was chairman of the Joint Intelligence Committee at the time of the 2003 invasion Getty Images The key players in the Iraq War Peter Goldsmith Peter Goldsmith was Mr Blair’s attorney general from 2001 to 2007 AFP/Getty Images The key players in the Iraq War Condoleezza Rice Condoleezza Rice was named as National Security Advisor to George W Bush in 2000, becoming the first woman to occupy the post, and argued publicly in favour of the 2003 invasion Getty Images The key players in the Iraq War Colin Powell Colin Powell was Secretary of State from 2001 to 2005 Getty Images for TIME The key players in the Iraq War Tommy Franks Tommy Franks was the leading US general at the start of the Iraq war The key players in the Iraq War Dick Cheney Dick Cheney was George W Bush’s vice president from 2001 to 2009 Getty Images The key players in the Iraq War Paul Bremer Paul Bremer ran Iraq for 14 months after the invasion, appointed Bush’s Presidential Envoy in charge of the occupying forces Getty Images The key players in the Iraq War Hans Blix Hans Blix was the UN weapons inspector tasked with monitoring Iraq from 2002 to 2003 Getty Images

Suicide bombers soon drove foreign embassies into the Green Zone where they remained as cut off from feelings and conditions in the rest of Iraq as if they had been stationed on a space ship hovering over the city. Whatever the level of failure nobody seemed to lose their job.