The framers of the UN charter gave the secretary-general two distinct functions: "chief administrative officer of the organisation; and an independent official whom the General Assembly and Security Council could entrust with certain unspecified (but implicitly political) tasks. Each holder of the office must demonstrate whether he is more secretary than general. Paradoxes abound. The secretary-general is expected to enjoy the backing of governments, especially those of the five permanent members of the Security Council, but be above partiality to any of them. He establishes his credentials by bureaucratic or diplomatic service but once elected must transcend his past and serve as a voice of the world, even a "secular pope".

The secretary-general is entrusted with assisting member states to make sound and well-informed decisions, which he is then obliged to execute, but he is also authorised to influence their work and even propose actions that they should undertake. He administers a complex organisation and serves as head of the UN agencies but must exercise this role within budgetary and regulatory constraints imposed by the member governments.

True, the secretary-general has an unparalleled agenda-shaping authority. But he does not have the power to execute all his ideas, and he articulates a vision that only governments can fulfil. He moves the world, but he cannot direct it.

It was Hammarskjold who, at the height of the Cold War, first argued that an impartial civil servant could be "politically celibate" without being "politically virgin". The secretary-general could play a political role without losing his impartiality, provided he hewed faithfully to the charter and international law.

Since the end of the Cold War, Kofi Annan has gone further than his predecessors in using his office as a "bully pulpit". He has boldly raised questions of the morality of intervention and the duty of the individual to follow his or her conscience and challenged member states to resolve the tensions between state sovereignty and their responsibility to protect ordinary people.

Often, a secretary-general can raise an awkward question but not dictate the appropriate answer. Annan's historic speech to the General Assembly in 1999 on intervention set "a thousand flowers blooming" at think-tanks and among op-ed columnists, but it has not led to a single military intervention to protect the oppressed. The UN is often seen embodying international legitimacy, yet the secretary-general's pronouncements often have less impact on the conduct of member states than the Pope's strictures on birth control.

The secretary-general knows that he can accomplish little without the support of members whose inaction on one issue or another he might otherwise want to denounce. He cannot afford to allow frustration on any one issue affect his ability to elicit cooperation from governments on a range of others. Annan once made that point by citing an old Ghanaian proverb: "Never hit a man on the head when you have your fingers between his teeth."

Today's single-superpower world also means the secretary-general must manage a relationship vital to the UN's survival without mortgaging his own integrity and independence. The insistent demands of some in the US that the UN prove its utility to America - demands that could not have been made in the same terms during the Cold War - oblige a secretary-general to walk a tightrope between heeding US priorities and the preferences of the membership as a whole. Paradoxically, he can be most useful to the US when he demonstrates his independence from it.

Member states' increasing micro-management of budgets has also weakened the secretary-general's authority. Both Annan and his predecessor, Boutros Boutros-Ghali, embarked on ambitious administrative reforms but were unable to address the far-greater levels of procedural and regulatory inertia in areas under the authority of the member states.

No secretary-general has enjoyed real independence from governments: the UN operates without embassies or intelligence services and member states resist any attempt to acquire such capabilities. A secretary-general's reach thus cannot exceed his grasp, and his grasp cannot extend across the member states' frontiers - or their pocketbooks.

Indeed, today the secretary-general commands great diplomatic legitimacy and even greater media visibility but less political power than the language of the UN charter suggests. To be effective, he must be skilled at managing staff and budgets, gifted at public diplomacy (and its behind-the-scenes variant) and able to engage the loyalties of a wide array of external actors, including non-governmental organisations, business groups and journalists.

He must also convince the nations of the poor and conflict-ridden South that their interests are uppermost in his mind, while ensuring that he can work effectively with the wealthy and powerful North. He must recognise the power and prerogatives of the Security Council, especially those of its five permanent members, while staying attentive to the priorities and passions of the General Assembly. And he must present member states with politically achievable proposals and implement his mandates within the means they provide him.

Above all, the secretary-general needs a vision of the higher purpose of his office and an awareness of its potential and limitations. In other words, to be successful, he must conceive and project a vision of the UN as it should be, while administering and defending the organisation as it is. Truly an impossible job.

Shashi Tharoor is a UN under-secretary-general and the author most recently of a collection of literary essays, "Bookless in Baghdad".

Copyright: Project Syndicate.

Shashi Tharoor