The deaths of the UN special representative to Iraq, Sergio Vieira de Mello, and his colleagues have severely dimmed the fragile hopes for peace and reconstruction that existed in recent weeks.
Yet irony being what it is, these tragic events also demonstrate just how much the United States and the United Nations need each other in Iraq, and just how real the opportunity is to spawn action, based on this common need.
In the three months they had been working together, the interaction between Vieira de Mello and US civil administrator Paul Bremer was surprisingly constructive.
While the solicitous relationship between the two career diplomats was not one of the more interesting aspects of the post-war situation, it was one of the more important because it created a real possibility that the international community could agree on a sustainable division of labour in Iraq.
It is now more important than ever that this opportunity is not wasted. Whether it is in the reconstruction of Iraq, the struggle against terrorism or the securing of peace in the Middle East - Vieira de Mello's death shows painfully that neither the US nor the UN can meet these challenges alone. Indeed, if it was not clear before this week, both sides should now understand how critical it is that they both pull together.
The Bush administration would be loath to admit it, but it may regret getting everything it wished for in security council resolution 1483, which gave almost exclusive authority for Iraq's reconstruction to the Americans and the British.
The post-war period has turned out much costlier than expected, in both human and financial terms, and it has presented qualitatively unanticipated challenges that the coalition is ill-equipped to address alone.
