Al-Majid - 'Chemical Ali' - is awaiting trial in Baghdad on charges of crimes against humanity, but his prosecution can hardly be said to represent a triumph of the international community's determination to make a stand against genocide when we are told every morning on the Today programme that the war that overthrew al-Majid's regime was 'illegal' precisely because it didn't have the support of the United Nations.

If you want to play judges and lawyers, the assertion that the lawful course was to leave Saddam in power seems grimly comic. Article I of the United Nations's 1951 Genocide Convention unequivocally states that 'contracting parties confirm that genocide, whether committed in time of peace or war, is a crime under international law which they undertake to prevent and punish.' What could be clearer? Admittedly, the major powers did not sign the convention straight away; the United States waited until 1988, when the poison gas campaign in Kurdistan was at its height, but since then, genocide has become a crime the whole world condemned.

Yet here's the rub. Since Kurdistan, genocide has increased. There have been campaigns of terror that were either full attempts at ethnic extermination or near enough as to make no difference in the former Yugoslavia from 1991 to 1995, Rwanda in 1994 and Darfur from 2002 to the present day. In all cases, the United Nations has promised to uphold the highest principles of international law and then committed sins of omission which were so grievous it has been close to being an accessory to mass murder.

The result is that any totalitarian regime or movement committing crimes against humanity knows it can get away with treating the UN with a derision that matches Chemical Ali's. On Thursday, a spokesman for the Sudanese government, whose militias have murdered about 400,000 in Darfur and driven two million from their homes, sounded like a psychopathic gangster when he announced that Jan Pronk, the UN's special envoy, was 'history'.

He explained that his government had expelled Pronk and would not allow him to return because he had been 'abusive' about human rights abuses.

The news of a direct attack on the authority of the United Nations produced no expressions of outrage. And although I think history will see the unwillingness to stop or even think about the genocide in Darfur as the great moral failure of our generation, I suppose that the indifference to the censoring and smearing of the UN's envoy is understandable. The genocide in Darfur has not been marked by a single dramatic event to focus the attention of the world on the Sudan, just a steady slaughter.

What may be less comprehensible to the uninitiated has been the reaction of the UN. Far from thumping the table and ordering the Sudanese ambassador to pack his bags and leave New York, the UN has been all soothing words. Even though it could no longer monitor a crime it professed to abhor, even though its representative had been stopped from doing his job, the UN's spokesman murmured: 'We need to take things one step at a time', then moved on to new business.

Complicity With Evil, Adam LeBor's riveting, if depressing, account of the UN's failure to act on the knowledge that mass murder is taking place, puts the spokesman's insouciance into a dismal context. The UN, particularly under the disastrous leadership of Kofi Annan, has three abiding faults that explain why it makes no attempt to live up to the high principles of its universal declarations of human rights.

1. It is a club without membership restrictions

Genocidal states aren't suspended from the UN or expelled. While they perpetrate crimes beyond the human imagination, their ambassadors remain honoured figures at the UN headquarters in Manhattan. Thus in 2004, the block votes of Arab and African dictatorships secured Sudan a place on the UN Commission on Human Rights, alongside the vile tyrannies of Libya and Zimbabwe. The fact that Sudan's militias were engaged in systematic murder, rape and looting did it no harm whatsoever and may well have boosted its electoral appeal.

2. It turns neutrality into a vice

Because no state is ever placed beyond the pale, the UN is 'reluctant to distinguish aggressor from victim' in the words of a highly critical internal inquiry into the role of its peacekeepers in the ethnic cleansing of Bosnia. Yasushi Akashi, the UN's special envoy in Bosnia, was as determined as Douglas Hurd and Boris Yeltsin not to offend Slobodan Milosevic. When British general Rupert Smith ordered the air strikes which finally lifted the siege of Sarajevo, he didn't bother to tell the UN, which would certainly have opposed them.

3. No one is held to account

Akashi was promoted after Bosnia and placed in charge of humanitarian affairs. Kofi Annan, who was not only in charge of the UN's peacekeeping operations during the Bosnian debacle but personally intervened to stop pre-emptive action to prevent the Rwandan genocide, was rewarded with the secretary-generalship and, perhaps unsurprisingly, then presided over the lavish corruption of the oil-for-food scandal which allowed Saddam to bribe supporters around the world.

Annan will be gone soon, but unless his successor can tackle the moral corruption of a potentially noble institution, then the UN should be honest with itself and world opinion and take Chemical Ali's words as its motto. 'Who is going to say anything? The international community? Fuck them!'