The murder of Sergio Vieira de Mello and his many United Nations colleagues on August 19 is a catastrophe. It is the international version of September 11, and it makes it infinitely harder for the UN to help in the reconstruction of an independent Iraq. Which is just what the murderers intended.
The numbers of dead bear no comparison. But the evil men who carried out this attack murdered more UN officials than in any other assault upon the organisation since it was created after the Second World War.
Just as September 11 was an attack upon America, August 19 was an attack, by the same sort of people, on the international system.
We have to see it as a direct assault upon the principles of international civil society that we have tried to create since 1945. It was an attempt to murder not just fine men and women ö the UN had sent its "A team" to Baghdad - but also the humane values that the UN, for all its shortcomings, represents and strives to fulfil.
Sergio Vieira de Mello was one of the most brilliant diplomats that the United Nations has ever produced. He was spoken of as a possible successor to Kofi Annan, who is widely thought to be the best leader the UN has ever had, but who is now suffering the murder of his friends and colleagues.
Among those murdered with Sergio were the superb, long-serving Nadia Younes of Egypt and such gifted young officials as Rick Hooper from the US, Fiona Watson from Britain, and Ranillo Buenaventura from the Philippines.
I had known Sergio for many years and watched him work in Cambodia, in the Balkans, in East Timor and in Africa. He was a joy to be with - a magnetic personality. He was intensely serious about his work, but he could also laugh and make fun of himself.
He was debonair, immaculate, remarkably handsome and had a smile that could launch a thousand ceasefires. Women adored him; men admired him.
A US senator once said:"Whenever I meet Sergio, two things happen. First, I feel poorly informed. Second, I feel poorly dressed." I have never heard anyone speak ill of him. People talked of "Sergio's magic".
In Cambodia in the early 1990s, he arranged the repatriation of 400,000 refugees with consummate skill, persuading recalcitrant communist officials to do as they were bid so charmingly.
In Bosnia, I went with him once to meet the egregious Serb leader, Radovan Karadzic. He was a psychiatrist of sorts and Sergio gave him the latest edition of the New York Review of Books, in which the cover story was about a war between psychiatrists.
Having charmed Karadzic, Sergio then sat down to hours of tough talks with him. We used to joke that the only possible title for his eventual memoirs was "My friends the war criminals". Now he will never write the book, because the war criminals got him.
In East Timor, he led the tiny community out of the wreckage left by Indonesia and into full independence. For this he and the UN were denounced by Osama bin Laden. Why? Because they had helped a basically Christian community secure its freedom from its Muslim occupiers.
There is speculation that his murder may have been the revenge of al-Qa'eda. But whoever committed this terrible crime was trying to stop the world giving Iraq a chance - its only chance - of decent government.
Sergio saw the UN's role as laying the foundations of civil society. He said: "The people of Iraq have suffered enough. It is time that we all... come together to ensure that this suffering comes to an end... We must not fail."
He said that he understood that the Iraqis did not want to be occupied. He quickly won the confidence of L Paul Bremer, the US administrator, and argued that Iraqis must be empowered as quickly as possible. He was a very good friend to the Iraqi people ö that is why the killers targeted him.
The murderers do not want the international community to succeed in building a decent Iraq - they want first chaos and then a new despotism, either Ba'athist or Islamic, to prevail.
Behind the horrible headlines the coalition has actually had important successes in Iraq. It set up the Iraqi Governing Council, which is far more representative than anything before in Iraq, and to which powers are being gradually handed over. It is beginning to set up a constitutional convention.
The coalition has started legal reforms to implement international human rights law. At least half of Baghdad's schools have now been reopened. Of Baghdad's 60 police stations, 34 are now operating. There is widespread support around the country.
But the coalition needs more international help. Kofi Annan has rightly said that the UN's work in Iraq must continue. Neither the UN nor the coalition can succeed unlethe battle for security is won. The light force that captured Baghdad so brilliantly in April is not strong enough to protect the entire country against those determined to sabotage its reconstruction.
America must provide more troops, despite the Pentagon's reluctance. Other countries should also be encouraged.
There are talks in New York about a new UN resolution to authorise hesitant countries such as India, Pakistan and Turkey to reinforce the coalition. But as American troops will remain by far the largest contingent, Washington is insisting on retaining overall control.
Mr Annan says that unlethe US is prepared to share authority it will be hard to get agreement on a second resolution. As before, the French are leading the objections. Some French officials seem to be taking a malicious pleasure in the predicament of "les Anglo-Saxons".
What the French and other critics of Bush and Blair should understand is that Iraq is a world problem, not an American or even an "Anglo-Saxon" one. Seventy per cent of Iraqi people recently polled said they welcomed the coalition's efforts to throw out Saddam and rebuild the country. They need everyone's help, not another nasty internecine war on the Security Council.
If the international effort to build a decent, democratic Iraq fails, the men who carried out September 11 and August 19 will have won a terrible victory. The only fitting memorial to Sergio and all those brave men and women who were murdered with him can be succeed in Iraq.
It is not only essential, it is possible.
William Shawcros is on the board of the International Crisis Group