And again I say: I do not disrespect the views of those in opposition to mine.

This is a tough choice. But it is also a stark one: to stand British troops down and turn back; or to hold firm to the course we have set.

I believe we must hold firm.

The question most often posed is not why does it matter? But: why does it matter so much? Here we are: the Government with its most serious test, its majority at risk, the first Cabinet resignation over an issue of policy. The main parties divided.

People who agree on everything else, disagree on this and likewise, those who never agree on anything, finding common cause.

The country and Parliament reflect each other: a debate that, as time has gone on has become less bitter but not less grave.

So: why does it matter so much?

Because the outcome of this issue will now determine more than the fate of the Iraqi regime and more than the future of the Iraqi people, for so long brutalised by Saddam. It will determine the way Britain and the world confront the central security threat of the 21st Century; the development of the UN; the relationship between Europe and the US; the relations within the EU and the way the US engages with the rest of the world. It will determine the pattern of international politics for the next generation.

But first, Iraq and its WMD.

In April 1991, after the Gulf War Iraq was given 15 days to provide a full and final declaration of all its WMD.

Saddam had used the weapons against Iran, against his own people, causing thousands of deaths. He had had plans to use them against allied forces. It became clear after the Gulf War that the WMD ambitions of Iraq were far more extensive than hitherto thought. This issue was identified by the UN as one for urgent remedy. UNSCOM, the weapons inspection team, was set up. They were expected to complete their task following the declaration at the end of April 1991.

The declaration when it came was false - a blanket denial of the programme, other than in a very tentative form. So the 12 year game began.

The inspectors probed. Finally in March 1992, Iraq admitted it had previously undeclared WMD but said it had destroyed them.

It gave another full and final declaration.

Again the inspectors probed but found little.

In October 1994, Iraq stopped co-operating with UNSCOM altogether. Military action was threatened. Inspections resumed.

In March 1995, in an effort to rid Iraq of the inspectors, a further full and final declaration of WMD was made. By July 1995, Iraq was forced to admit that too was false. In August they provided yet another full and final declaration.

Then, a week later, Saddam's son-in-law, Hussein Kamal defected to Jordan. He disclosed a far more extensive BW programme and for the first time said Iraq had weaponised the programme; something Saddam had always strenuously denied. All this had been happening whilst the inspectors were in Iraq. Kamal also revealed Iraq's crash programme to produce a nuclear weapon in 1990.

Iraq was forced then to release documents which showed just how extensive those programmes were.

In November 1995, Jordan intercepted prohibited components for missiles that could be used for WMD.

In June 1996, a further full and final declaration was made.

That too turned out to be false.

In June 1997, inspectors were barred from specific sites.

In September 1997, another full and final declaration was made. Also false. Meanwhile the inspectors discovered VX nerve agent production equipment, something always denied by the Iraqis.

In October 1997, the US and the UK threatened military action if Iraq refused to comply with the inspectors. But obstruction continued.

Finally, under threat of action, in February 1998, Kofi Annan went to Baghdad and negotiated a memorandum with Saddam to allow inspections to continue.

They did.

For a few months.

In August, co-operation was suspended.

In December the inspectors left. Their final report is a withering indictment of Saddam's lies, deception and obstruction, with large quantities of WMD remained unaccounted for. The US and the UK then, in December 1998, undertook Desert Fox, a targeted bombing campaign to degrade as much of the Iraqi WMD facilities as we could.

In 1999, a new inspections team, UNMOVIC, was set up. But Saddam refused to allow them to enter Iraq.

So there they stayed, in limbo, until after Resolution 1441 when last November they were allowed to return.

What is the claim of Saddam today?

Why exactly the same claim as before: that he has no WMD.

Indeed we are asked to believe that after seven years of obstruction and non-compliance finally resulting in the inspectors leaving in 1998, seven years in which he hid his programme, built it up even whilst inspection teams were in Iraq, that after they left he then voluntarily decided to do what he had consistently refused to do under coercion.

When the inspectors left in 1998, they left unaccounted for:

10 thousand litres of anthrax a far reaching VX nerve agent programme up to 6,500 chemical munitions at least 80 tonnes of mustard gas, possibly more than ten times that amount unquantifiable amounts of sarin, botulinum toxin and a host of other biological poisons an entire Scud missile programme

We are now seriously asked to accept that in the last few years, contrary to all history, contrary to all intelligence, he decided unilaterally to destroy the weapons. Such a claim is palpably absurd.

1441 is a very clear Resolution. It lays down a final opportunity for Saddam to disarm. It rehearses the fact that he has been, for years in material breach of 17 separate UN Resolutions.

It says that this time compliance must be full, unconditional and immediate. The first step is a full and final declaration of all WMD to be given on 8 December.

I won't to go through all the events since then - the House is familiar with them - but this much is accepted by all members of the UNSC.

The 8 December declaration is false. That in itself is a material breach.

Iraq has made some concessions to co-operation but no-one disputes it is not fully co-operating.

Iraq continues to deny it has any WMD, though no serious intelligence service anywhere in the world believes them.

On 7 March, the inspectors published a remarkable document. It is 173 pages long, detailing all the unanswered questions about Iraq's WMD. It lists 29 different areas where they have been unable to obtain information.

For example, on VX it says: "Documentation available to UNMOVIC suggests that Iraq at least had had far reaching plans to weaponise VX ...

"Mustard constituted an important part (about 70%) of Iraq's CW arsenal ... 550 mustard filled shells and up to 450 mustard filled aerial bombs unaccounted for ... additional uncertainty with respect of 6526 aerial bombs, corresponding to approximately 1000 tonnes of agent, predominantly mustard."

"Based on unaccounted for growth media, Iraq's potential production of anthrax could have been in the range of about 15,000 to 25,000 litres ... Based on all the available evidence, the strong presumption is that about 10,000 litres of anthrax was not destroyed and may still exist."

On this basis, had we meant what we said in Resolution 1441, the Security Council should have convened and condemned Iraq as in material breach.

What is perfectly clear is that Saddam is playing the same old games in the same old way.

Yes there are concessions. But no fundamental change of heart or mind.

But the inspectors indicated there was at least some co-operation; and the world rightly hesitated over war. We therefore approached a second Resolution in this way.

We laid down an ultimatum calling upon Saddam to come into line with Resolution 1441 or be in material breach.

Not an unreasonable proposition, given the history.

But still countries hesitated: how do we know how to judge full co-operation?

We then worked on a further compromise. We consulted the inspectors and drew up five tests based on the document they published on 7 March. Tests like interviews with 30 scientists outside of Iraq; production of the anthrax or documentation showing its destruction.

The inspectors added another test: that Saddam should publicly call on Iraqis to co-operate with them.

So we constructed this framework: that Saddam should be given a specified time to fulfil all six tests to show full co-operation; that if he did so the inspectors could then set out a forward work programme and that if he failed to do so, action would follow.

So clear benchmarks; plus a clear ultimatum.

I defy anyone to describe that as an unreasonable position.

Last Monday, we were getting somewhere with it. We very nearly had majority agreement and I thank the Chilean President particularly for the constructive way he approached the issue.

There were debates about the length of the ultimatum. But the basic construct was gathering support.

Then, on Monday night, France said it would veto a second Resolution whatever the circumstances.

Then France denounced the six tests. Later that day, Iraq rejected them.

Still, we continued to negotiate.

Last Friday, France said they could not accept any ultimatum. On Monday, we made final efforts to secure agreement. But they remain utterly opposed to anything which lays down an ultimatum authorising action in the event of non-compliance by Saddam.

Just consider the position we are asked to adopt. Those on the Security Council opposed to us say they want Saddam to disarm but will not countenance any new Resolution that authorises force in the event of non-compliance.

That is their position. No to any ultimatum; no to any Resolution that stipulates that failure to comply will lead to military action.

So we must demand he disarm but relinquish any concept of a threat if he doesn't. From December 1998 to December 2002, no UN inspector was allowed to inspect anything in Iraq. For four years, not a thing. What changed his mind? The threat of force. From December to January and then from January through to February, concessions were made.

What changed his mind?

The threat of force.

And what makes him now issue invitations to the inspectors, discover documents he said he never had, produce evidence of weapons supposed to be non-existent, destroy missiles he said he would keep? The imminence of force.

The only persuasive power to which he responds is 250,000 allied troops on his doorstep. And yet when that fact is so obvious that it is staring us in the face, we are told that any Resolution that authorises force will be vetoed.

Not just opposed. Vetoed. Blocked.

The way ahead was so clear. It was for the UN to pass a second Resolution setting out benchmarks for compliance; with an ultimatum that if they were ignored, action would follow.

The tragedy is that had such a Resolution issued, he might just have complied. Because the only route to peace with someone like Saddam Hussein is diplomacy backed by force. Yet the moment we proposed the benchmarks, canvassed support for an ultimatum, there was an immediate recourse to the language of the veto.

And now the world has to learn the lesson all over again that weakness in the face of a threat from a tyrant, is the surest way not to peace but to war.

Looking back over 12 years, we have been victims of our own desire to placate the implacable, to persuade towards reason the utterly unreasonable, to hope that there was some genuine intent to do good in a regime whose mind is in fact evil. Now the very length of time counts against us. You've waited 12 years. Why not wait a little longer?

And indeed we have.

1441 gave a final opportunity. The first test was the 8th of December. He failed it.

But still we waited. Until the 27th of January, the first inspection report that showed the absence of full co-operation. Another breach. And still we waited.

Until the 14th of February and then the 28th of February with concessions, according to the old familiar routine, tossed to us to whet our appetite for hope and further waiting. But still no-one, not the inspectors nor any member of the Security Council, not any half-way rational observer, believes Saddam is co-operating fully or unconditionally or immediately.

Our fault has not been impatience.

The truth is our patience should have been exhausted weeks and months and years ago. Even now, when if the world united and gave him an ultimatum: comply or face forcible disarmament, he might just do it, the world hesitates and in that hesitation he senses the weakness and therefore continues to defy.

What would any tyrannical regime possessing WMD think viewing the history of the world's diplomatic dance with Saddam? That our capacity to pass firm resolutions is only matched by our feebleness in implementing them.

That is why this indulgence has to stop. Because it is dangerous. It is dangerous if such regimes disbelieve us. Dangerous if they think they can use our weakness, our hesitation, even the natural urges of our democracy towards peace, against us. Dangerous because one day they will mistake our innate revulsion against war for permanent incapacity; when in fact, pushed to the limit, we will act. But then when we act, after years of pretence, the action will have to be harder, bigger, more total in its impact. Iraq is not the only regime with WMD. But back away now from this confrontation and future conflicts will be infinitely worse and more devastating.

But, of course, in a sense, any fair observer does not really dispute that Iraq is in breach and that 1441 implies action in such circumstances. The real problem is that, underneath, people dispute that Iraq is a threat; dispute the link between terrorism and WMD; dispute the whole basis of our assertion that the two together constitute a fundamental assault on our way of life.

There are glib and sometimes foolish comparisons with the 1930s. No-one here is an appeaser. But the only relevant point of analogy is that with history, we know what happened. We can look back and say: there's the time; that was the moment; for example, when Czechoslovakia was swallowed up by the Nazis - that's when we should have acted. But it wasn't clear at the time. In fact at the time, many people thought such a fear fanciful. Worse, put forward in bad faith by warmongers. Listen to this editorial - from a paper I'm pleased to say with a different position today - but written in late 1938 after Munich when by now, you would have thought the world was tumultuous in its desire to act.

"Be glad in your hearts. Give thanks to your God. People of Britain, your children are safe. Your husbands and your sons will not march to war. Peace is a victory for all mankind. And now let us go back to our own affairs. We have had enough of those menaces, conjured up from the Continent to confuse us."

Naturally should Hitler appear again in the same form, we would know what to do. But the point is that history doesn't declare the future to us so plainly. Each time is different and the present must be judged without the benefit of hindsight.

So let me explain the nature of this threat as I see it.

The threat today is not that of the 1930s. It's not big powers going to war with each other. The ravages which fundamentalist political ideology inflicted on the 20th century are memories.

The Cold War is over.

Europe is at peace, if not always diplomatically.

But the world is ever more interdependent. Stock markets and economies rise and fall together.

Confidence is the key to prosperity.

Insecurity spreads like contagion.

So people crave stability and order.

The threat is chaos.

And there are two begetters of chaos.

Tyrannical regimes with WMD and extreme terrorist groups who profess a perverted and false view of Islam.

Let me tell the House what I know.

I know that there are some countries or groups within countries that are proliferating and trading in WMD, especially nuclear weapons technology.

I know there are companies, individuals, some former scientists on nuclear weapons programmes, selling their equipment or expertise.

I know there are several countries - mostly dictatorships with highly repressive regimes - desperately trying to acquire chemical weapons, biological weapons or, in particular, nuclear weapons capability. Some of these countries are now a short time away from having a serviceable nuclear weapon. This activity is not diminishing. It is increasing. We all know that there are terrorist cells now operating in most major countries. Just as in the last two years, around 20 different nations have suffered serious terrorist outrages.

Thousands have died in them.

The purpose of terrorism lies not just in the violent act itself. It is in producing terror. It sets out to inflame, to divide, to produce consequences which they then use to justify further terror.

Round the world it now poisons the chances of political progress: in the Middle East; in Kashmir; in Chechnya; in Africa.

The removal of the Taliban in Afghanistan dealt it a blow. But it has not gone away. And these two threats have different motives and different origins but they share one basic common view: they detest the freedom, democracy and tolerance that are the hallmarks of our way of life.

At the moment, I accept that association between them is loose. But it is hardening. And the possibility of the two coming together - of terrorist groups in possession of WMD, even of a so-called dirty radiological bomb is now, in my judgement, a real and present danger.

And let us recall: what was shocking about 11 September was not just the slaughter of the innocent; but the knowledge that had the terrorists been able to, there would have been not 3,000 innocent dead, but 30,000 or 300,000 and the more the suffering, the greater the terrorists' rejoicing.

3 kilograms of VX from a rocket launcher would contaminate a quarter of a square kilometre of a city.

Millions of lethal doses are contained in one litre of Anthrax. 10,000 litres are unaccounted for.

11 September has changed the psychology of America. It should have changed the psychology of the world.

Of course Iraq is not the only part of this threat. But it is the test of whether we treat the threat seriously.

Faced with it, the world should unite. The UN should be the focus, both of diplomacy and of action. That is what 1441 said. That was the deal. And I say to you to break it now, to will the ends but not the means that would do more damage in the long term to the UN than any other course.

To fall back into the lassitude of the last 12 years, to talk, to discuss, to debate but never act; to declare our will but not enforce it; to combine strong language with weak intentions, a worse outcome than never speaking at all.

And then, when the threat returns from Iraq or elsewhere, who will believe us? What price our credibility with the next tyrant? No wonder Japan and South Korea, next to North Korea, has issued such strong statements of support.

I have come to the conclusion after much reluctance that the greater danger to the UN is inaction: that to pass Resolution 1441 and then refuse to enforce it would do the most deadly damage to the UN's future strength, confirming it as an instrument of diplomacy but not of action, forcing nations down the very unilateralist path we wish to avoid. But there will be, in any event, no sound future for the UN, no guarantee against the repetition of these events, unless we recognise the urgent need for a political agenda we can unite upon.

What we have witnessed is indeed the consequence of Europe and the United States dividing from each other. Not all of Europe - Spain, Italy, Holland, Denmark, Portugal - have all strongly supported us. And not a majority of Europe if we include, as we should, Europe's new members who will accede next year, all 10 of whom have been in our support.

But the paralysis of the UN has been born out of the division there is. And at the heart of it has been the concept of a world in which there are rival poles of power. The US and its allies in one corner. France, Germany, Russia and its allies in the other. I do not believe that all of these nations intend such an outcome. But that is what now faces us.

I believe such a vision to be misguided and profoundly dangerous. I know why it arises. There is resentment of US predominance. There is fear of US unilateralism. People ask: do the US listen to us and our preoccupations? And there is perhaps a lack of full understanding of US preoccupations after 11th September. I know all of this.

But the way to deal with it is not rivalry but partnership. Partners are not servants but neither are they rivals. I tell you what Europe should have said last September to the US. With one voice it should have said: we understand your strategic anxiety over terrorism and WMD and we will help you meet it. We will mean what we say in any UN Resolution we pass and will back it with action if Saddam fails to disarm voluntarily; but in return we ask two things of you: that the US should choose the UN path and you should recognise the fundamental overriding importance of re-starting the MEPP, which we will hold you to.

I do not believe there is any other issue with the same power to re-unite the world community than progress on the issues of Israel and Palestine. Of course there is cynicism about recent announcements. But the US is now committed, and, I believe genuinely, to the Roadmap for peace, designed in consultation with the UN. It will now be presented to the parties as Abu Mazen is confirmed in office, hopefully today.

All of us are now signed up to its vision: a state of Israel, recognised and accepted by all the world, and a viable Palestininan state.

And that should be part of a larger global agenda. On poverty and sustainable development. On democracy and human rights. On the good governance of nations. That is why what happens after any conflict in Iraq is of such critical significance. Here again there is a chance to unify around the UN. Let me make it clear.

There should be a new UN Resolution following any conflict providing not just for humanitarian help but also for the administration and governance of Iraq. That must now be done under proper UN authorisation.

It should protect totally the territorial integrity of Iraq.

And let the oil revenues - which people falsely claim we want to seize - be put in a Trust fund for the Iraqi people administered through the UN.

And let the future government of Iraq be given the chance to begin the process of uniting the nation's disparate groups, on a democratic basis, respecting human rights, as indeed the fledgling democracy in Northern Iraq - protected from Saddam for 12 years by British and American pilots in the No Fly Zone - has done so remarkably.

And the moment that a new government is in place - willing to disarm Iraq of WMD - for which its people have no need or purpose - then let sanctions be lifted in their entirety. I have never put our justification for action as regime change. We have to act within the terms set out in Resolution 1441. That is our legal base.

But it is the reason, I say frankly, why if we do act we should do so with a clear conscience and strong heart.

I accept fully that those opposed to this course of action share my detestation of Saddam. Who could not? Iraq is a wealthy country that in 1978, the year before Saddam seized power, was richer than Portugal or Malaysia.

Today it is impoverished, 60% of its population dependent on Food Aid.

Thousands of children die needlessly every year from lack of food and medicine. Four million people out of a population of just over 20 million are in exile. The brutality of the repression - the death and torture camps, the barbaric prisons for political opponents, the routine beatings for anyone or their families suspected of disloyalty are well documented.

Just last week, someone slandering Saddam was tied to a lamp post in a street in Baghdad, his tongue cut out, mutilated and left to bleed to death, as a warning to others. I recall a few weeks ago talking to an Iraqi exile and saying to her that I understood how grim it must be under the lash of Saddam.

"But you don't", she replied. "You cannot. You do not know what it is like to live in perpetual fear." And she is right. We take our freedom for granted. But imagine not to be able to speak or discuss or debate or even question the society you live in. To see friends and family taken away and never daring to complain. To suffer the humility of failing courage in face of pitiless terror. That is how the Iraqi people live. Leave Saddam in place and that is how they will continue to live.

We must face the consequences of the actions we advocate. For me, that means all the dangers of war. But for others, opposed to this course, it means - let us be clear - that the Iraqi people, whose only true hope of liberation lies in the removal of Saddam, for them, the darkness will close back over them again; and he will be free to take his revenge upon those he must know wish him gone.

And if this House now demands that at this moment, faced with this threat from this regime, that British troops are pulled back, that we turn away at the point of reckoning, and that is what it means - what then?

What will Saddam feel? Strengthened beyond measure. What will the other states who tyrannise their people, the terrorists who threaten our existence, what will they take from that? That the will confronting them is decaying and feeble. Who will celebrate and who will weep?

And if our plea is for America to work with others, to be good as well as powerful allies, will our retreat make them multilateralist? Or will it not rather be the biggest impulse to unilateralism there could ever be. And what of the UN and the future of Iraq and the MEPP, devoid of our influence, stripped of our insistence?

This House wanted this decision. Well it has it. Those are the choices. And in this dilemma, no choice is perfect, no cause ideal.

But on this decision hangs the fate of many things.

I can think of many things, of whether we summon the strength to recognise the global challenge of the 21st century and beat it, of the Iraqi people groaning under years of dictatorship, of our armed forces - brave men and women of whom we can feel proud, whose morale is high and whose purpose is clear - of the institutions and alliances that shape our world for years to come.

To retreat now, I believe, would put at hazard all that we hold dearest, turn the United Nations back into a talking shop, stifle the first steps of progress in the Middle East; leave the Iraqi people to the mercy of events on which we would have relinquished all power to influence for the better.

Tell our allies that at the very moment of action, at the very moment when they need our determination that Britain faltered. I will not be party to such a course. This is not the time to falter. This is the time for this House, not just this government or indeed this Prime Minister, but for this House to give a lead, to show that we will stand up for what we know to be right, to show that we will confront the tyrannies and dictatorships and terrorists who put our way of life at risk, to show at the moment of decision that we have the courage to do the right thing.