On March 20, 2003, prime minister Tony Blair ordered British Forces into action against Iraq after telling us Saddam Hussein possessed weapons of mass destruction that posed a threat to Britain and the world.

Thirteen years later, we know the full consequences of Mr Blair’s decision: 179 brave British servicemen and women were killed, and hundreds more were maimed or suffered psychological trauma.

Tens of thousands of Iraqis died as the fall of Saddam led to a vicious sectarian war between the country’s Shia majority and Sunni minority.

The way in which Tony Blair’s government took us to war in so dishonest a fashion surely marks 2003 as the point when the British people’s already shaky faith in the political class began to crumble into dust

President Barack Obama has pointed out that the destruction of Saddam has led to the rise of Islamic State, the most vicious and terrifying terror group the world has known.

So Iraq has turned into one of the greatest disasters in modern history. It is a far bigger error than Sir Anthony Eden’s infamous decision to attempt to reclaim the Suez Canal from Egypt in 1956.

In retrospect, it can be seen as the worst mistake in British foreign policy since Neville Chamberlain struck his notorious deal with Hitler in Munich in 1938.

Indeed, the way in which Tony Blair’s government took us to war in so dishonest a fashion surely marks 2003 as the point when the British people’s already shaky faith in the political class began to crumble into dust.

On March 20, 2003, prime minister Tony Blair ordered British Forces into action against Iraq after telling us Saddam Hussein. He is pictured with U.S. President George Bush (left) in 2003

Now, we need desperately to learn the lessons. And yet for 13 years, the British establishment has covered up the truth about Iraq.

There have been four botched inquiries (including the infamous judicial report chaired by Lord Hutton into the still mysterious death of David Kelly, the government scientist who was found dead in July 2003 after being exposed as the source of claims by the BBC journalist Andrew Gilligan that New Labour had ‘sexed up’ the threat from Saddam Hussein).

The fifth inquiry, Sir John Chilcot’s report, was announced by the then prime minister Gordon Brown in June 2009.

It was meant to reach a definitive judgment on the war — and was supposed to last for little more than a year. Then it ran into the sands of bureaucratic inertia and obstruction.

Tomorrow, this monumental investigation, which is said to run to 2.6 million words — almost five times longer than Tolstoy’s War And Peace — will finally see the light of day. Yet there are already reasons to doubt whether Sir John is capable of reaching the fair-minded verdict that will enable the nation — and above all the families of the heroes who died serving their country — to put the Iraq tragedy behind us.

According to advance leaks, Sir John will apportion the blame very widely with several dozen ministers, officials and military figures coming in for criticism.

These same reports also suggest a great deal of the report will concentrate on mistakes made during the occupation of Iraq rather than the decision to go to war in the first place.

If these leaks are true, they suggest that Sir John’s report, like the whitewashed reports that have already been published, will lack focus.

There are in truth only a handful of crucial points it should address. Did Tony Blair lie in order to make the case for war? Was the war legal?

Did the war — as Tony Blair promised it would — make Britain a safer place?

Crucially, the vast bulk of the evidence presented to Chilcot is available for all to see.

This means that any careful and well-informed observer can reach his or her own conclusions.

The Iraq war expert Dr David Morrison and I have spent months poring through the evidence to show the conclusions that Sir John Chilcot must surely make if he is true to the evidence put before him.

Let’s start with the question of whether Tony Blair deceived the British people in order to make the case for war.

Mr Blair has consistently asserted that he did not lie and that he acted in good faith.

For example, the former prime minister told the American TV channel CNN that he apologised ‘for the fact that the intelligence we received was wrong’.

Here, Mr Blair was placing the blame on the British intelligence services for producing erroneous information which he, as prime minister, innocently passed on to the British public.

This account of events does not, however, stand up to scrutiny. It is very easy to prove Mr Blair did not simply reiterate what he was told by the intelligence services.

The fact is he exaggerated and misrepresented the intelligence that he received from the Joint Intelligence Committee.

Evidence available to Sir John Chilcot shows Joint Intelligence Committee assessments sent to the prime minister about the threat posed by Iraqi weapons programmes were cautious.

They stated that ‘intelligence of Iraq’s weapons of mass destruction and ballistic missile programmes is sporadic and patchy’.

By contrast, Tony Blair claimed ‘we know that he [Saddam Hussein] had stockpiles of major amounts of chemical and biological weapons, we know that he is trying to acquire nuclear capability, we know that he is trying to develop ballistic capability of a greater range’.

According to advance leaks, Sir John will apportion the blame very widely with several dozen ministers, officials and military figures coming in for criticism

In truth, evidence presented to the Chilcot inquiry shows that the then prime minister repeatedly exaggerated and made false claims about the threat posed by Saddam as the war approached.

He did so in the notorious dossier on the threat posed by Saddam, published on September 24, 2002.

Incredibly, he lied to Parliament in his famous speech on March 18, 2003, on the very eve of war.

In this address, Mr Blair systematically distorted the work of the United Nations weapons inspectors, who at the start of March had published a document on the state of knowledge of Iraq’s weapons of mass destruction.

Tony Blair and his foreign secretary, Jack Straw, used this document shamelessly, giving the impression that it contained new and damning evidence that Iraq possessed proscribed weapons.

In reality, it contained little or nothing new.

Crucially, it did not claim that Iraq possessed dangerous weapons or weapons-related materials, merely that certain material was unaccounted for.

Mr Blair cited a claim by Saddam Hussein’s son-in-law, Hussein Kamel, that Saddam had an extensive biological weapons programme.

Shockingly, Mr Blair did not divulge to Parliament that Kamel also said that all Iraq’s weapons had been destroyed.

In truth, evidence presented to the Chilcot inquiry shows that the then prime minister repeatedly exaggerated and made false claims about the threat posed by Saddam as the war approached

To sum up, Tony Blair deliberately deceived the British people and Parliament in order to substantiate his decision to go to war.

We will not know until tomorrow whether Sir John Chilcot will conclude that Blair lied. But what we can say for certain is that a mountain of evidence exists that the British prime minister did exactly that.

The second crucial judgment facing Chilcot is whether the war was illegal — though some already have grave doubts that he will answer this point definitively.

Yet here again the evidence before him is unambiguous.

It is a fundamental principle of international law that states are prohibited from using force except in self-defence or unless its use is formally authorised by the UN Security Council.

No country was attacked by Iraq in March 2003, and therefore there were no grounds to go to war with Iraq on the grounds of self-defence.

The Security Council never authorised military action to disarm Iraq of its so-called ‘weapons of mass destruction’.

However, at most four of the 15 members of the UN Security Council were in favour of military action against Iraq in March 2003. Every single member of the British Foreign Office legal department was convinced the war was unlawful.

There is therefore no serious doubt that the attack on Iraq by the United States and the UK in March 2003 was illegal, and therefore a war of aggression.

We will not know until tomorrow whether Sir John Chilcot will conclude that Blair lied but a mountain of evidence exists that he did

The evidence that Tony Blair lied to Parliament and the British people in order to make the case for an illegal war is very strong indeed. Very serious and senior police and former officials are convinced this was the case. They include Hans Blix, the chief UN weapons inspector at the time of the Iraq invasion.

When I asked Mr Blix whether he believed that Tony Blair had misrepresented the facts to make the case for an illegal invasion, he said that he did.

Very few people deserve to emerge well from the Iraq Inquiry. However, I believe that one of them should be Eliza Manningham-Buller, the then Director General of MI5, the domestic intelligence service.

Dame Eliza told Chilcot that she warned Tony Blair in advance that the invasion of Iraq would increase the threat to Britain from the terrorist group Al-Qaeda, and radicalise increasing numbers of British Muslims.

When she appeared in front of the Iraq Inquiry, she was asked: ‘To what extent did the conflict in Iraq exacerbate the overall threat that your service and your fellow services were having to deal with from international terrorism?’

She replied: ‘Substantially.’ After the publication of the report, many people will want to know whether Mr Blair can be tried as a war criminal. This question is well worth considering. After World War II, the victors established an international military tribunal at Nuremberg to try leading Nazis.

President Barack Obama has pointed out that the destruction of Saddam has led to the rise of Islamic State, the most vicious and terrifying terror group the world has known

Article Six of the tribunal’s constitution specified the crimes falling within its jurisdiction. First and foremost was the ‘planning, preparation, initiation or waging of a war of aggression’.

As a prime mover in the invasion of Iraq, Blair could reasonably be accused of committing such a crime. So, too, could the then U.S. President George W. Bush.

In theory, this means that there’s a case for putting Blair on trial as a war criminal.

But the only body that could conceivably have tried him was the International Criminal Court (ICC) in The Hague.

At the time of the 2003 Iraq invasion, the UK was a party to the ICC, which had begun operating in July 2002.

It had the power to try even heads of state for genocide, war crimes or crimes against humanity. But as Lord Goldsmith, the Labour Attorney General, had advised Blair just before the Iraq invasion, the court had ‘no jurisdiction over the crime of aggression’.

Therefore, in leading the UK to war against Iraq, Blair knew he had nothing to worry about from the ICC.

As a prime mover in the invasion of Iraq, Blair could reasonably be accused of committing such a crime. So, too, could the then U.S. President George W. Bush (left)

(How bitterly ironic that Downing Street is now having to warn the court in The Hague that Chilcot’s report should not be used as a basis to prosecute British soldiers — never mind the man who sent them to war.)

While we will learn more about the actions of Tony Blair tomorrow, we must not forget how the way he behaved has influenced politicians who have come after him.

The delay in the publication of the Chilcot Inquiry has had deadly consequences, because the British government has continued to make the same mistakes that were made in the run-up to the Iraq invasion.

As Chilcot’s inquiry has dragged on, British forces have been involved in fresh foreign engagements in Libya, in 2011, and Syria more recently, carrying out bombing missions in both countries, as well as being drawn back into Iraq.

The lessons of the Iraq War — as set out in an official inquiry — would have been relevant in all these cases.

By A dark irony, the timing of the Chilcot report now that it’s finally with us could not be more appropriate. It comes against the sombre background of the total collapse of trust in the governing class.

This came to a head in the decision of the British people to defy all the main political party leaders and vote Britain out of Europe two weeks ago.

Faith in politics has never been lower, and this collapse in trust can be dated very precisely to the decision to go to war in 2003.

The British people and Parliament believed the claims made by its most senior politicians and foreign policy advisers.

It followed them blindly to war, with consequences we still have to live with today.

This is why the Chilcot Inquiry matters a great deal. It is the last chance for the British Establishment to show it can learn the lessons of its failures — and hold those who fail to account.

If Sir John Chilcot and his inquiry fail to achieve this, it will be the final proof that our system of government is broken.