There is an old, perhaps apocryphal story of a small girl who, watching the ranting, gesticulating Randolph Churchill, tugged at her mother's skirt and asked: “Mummy, what is that man for?”

The same must now be asked of the United Nations. The failure of the Security Council to agree a set of modest sanctions against Zimbabwe and Robert Mugabe's henchmen - such as a freeze on financial assets and a travel ban - speaks volumes about the reality of the UN and the fatuity of those who place any moral store by its decisions.

There could be no clearer case for action. No civilised nation can regard Mr Mugabe's behaviour as anything other than obscene. But decisions of the Security Council have never been based on decency or morality. They are based on realpolitik. The UN's very constitution as a body including some of the most brutal dictatorships on the planet necessitates that.

Indeed, the UN is structurally incapable of acting in accordance with the dictates of civilised behaviour. Whether it is its failure to stand up to the Burmese regime or to deal with the threat to Israel posed by a nuclear Iran, or its support for Hezbollah, the UN has shown itself to be not the promoter but the enemy of human rights.

The most bizarre reaction to the Security Council's rejection of sanctions is disappointment. Could anyone seriously expect the Chinese Government, which locks up and tortures dissidents and props up the Mugabe regime to further its own economic interests, to overturn decades of foreign policy and act in support of democracy and human rights? In 2005 the Chinese signed an aid agreement with Zimbabwe and made an explicit promise not to interfere in its “internal affairs”, saying that it “trusts Zimbabwe's Government and people have the ability to deal properly with their own matters”.

The idea that the UN holds some special legitimacy and moral worth is not merely naive - it can make a bad situation worse. Mugabe now claims that he has been exonerated by the UN. Had the UN not existed, no attention would be paid to the failure of Russia and China to criticise him, because that is entirely to be expected. And if, as they should, the EU's member states were to impose stronger sanctions, that would not be seen as somehow in opposition to the UN.

The UN has never had greater moral legitimacy than any other ad hoc assemblage of states. Far more legitimacy would attach to a league of democracies, as suggested by the US presidential candidate John McCain. Its decisions would have the moral force of democratic backing. It is time to say goodbye to the moral bankruptcy of the UN.

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