Dag Hammarskjöld, who died 50 years ago this week, led the United Nations as it has never been led before or since. He elevated the role of UN secretary general and affirmed its impartiality in the face of superpowers. He exploited the UN's legitimacy in the service of peace, inventing the new arts of peacekeeping and mediation. Some speculate about the plane crash that killed him, but the manner of his death is hardly as significant as the example of his life. On arriving at the UN, he streamlined the secretariat, something nobody else has yet achieved. His warning that violating the independence of the international service would be "the Munich of international co-operation" should sound an alarm today. Then he began ameliorating crises. First, Chou En-Lai was charmed and American hostages in China were freed. Later, a UN force was improvised to fix Suez. Finally, he stepped into the Congo, a vast and coveted country which a brutal colonial regime had left overnight. Hammarskjöld followed knowledge like the sinking star, immersing himself not only in languages but literatures. When he convened nuclear scientists, he could keep up. His bestselling Markings testifies to a rich, if anguished, solitude. And his marriage of the active and the contemplative life would have impressed a Cicero or a Seneca. As noted above, world leaders fly to New York this week for the UN general assembly. The intellect, integrity and idealism of Hammarskjöld's example should serve both as a reproach and an inspiration.