The Best Intentions, by James Traub/ Complicity with Evil, by Adam LeBor

Peux ce que veux: where there's will, there's a way. These fateful words of the courageous UN force commander in Rwanda could be the motto of the United Nations. Unfortunately, in the light of the Darfur tragedy, among others, they will not define the legacy of Kofi Annan.

On 11 January 1994, General Roméo Dallaire sent what has become known as the "genocide fax" to the UN peacekeeping department, then headed by Annan, warning of a looming mass slaughter of the Tutsi minority. He ended his message "Peux ce que veux". Annan's response: "We must handle this information with caution." Four months later, the killing of almost one million Rwandans began.

As Annan prepares to leave office, the first two books to scrutinise his record ask whether the lessons of Bosnia and Rwanda have been learnt. Both conclude that the UN cannot prevent genocide. They retread the genocidal events of the last 15 years, and the slow-motion genocide in Darfur, and of course point the finger of blame at the big players in the Security Council for lacking political will. But all of these upheavals happened on Annan's watch. As secretary-general, he had the opportunity to seize the high moral ground. On that front, with some notable exceptions, the genial Ghanaian who rose through the UN ranks has been found sadly lacking.

James Traub's book on the UN in the age of American hegemony quotes Bill Clinton's aide Nancy Soderberg, who addresses the Rwandan genocide in the light of the trauma of the "Black Hawk Down" incident in Somalia, which left 18 US Rangers dead and paralysed American peacekeeping for years. For her, "The ugly truth is that at the root of the failure to act is a belief [that the US] has little responsibility to protect the lives of the victims of an ongoing genocide". Now, 12 years after Somalia, and despite a declaration by the then secretary of state Colin Powell that atrocities in Darfur by the Sudanese government and its allied militias constitutes genocide, this analysis still holds true. This time it is Iraq that has stymied the international response.

If states are not prepared to intervene to stop crimes against humanity, despite the ringing declarations of "never again", who is? That is where the buck stops with the UN and its leader. Traub, a New York Times contributor who gained access to Annan and his top aides after accompanying the secretary-general on his peace mission to Baghdad in 1998, does not do a hatchet job. He deftly wields a stiletto, which he buries deep between the shoulder blades of the world's top diplomat.

Annan is depicted as a cipher, an empty vessel into which world leaders pour their own desires. Traub describes a meeting in July 2004 in El Fashier, Darfur, between Annan and Sudanese government representatives - four months after a senior UN official in Khartoum, Mukesh Kapila, publicly warned on the Today programme that a great humanitarian catastrophe was unfolding, on a par with Rwanda, and that the Sudanese government was behind it. He had issued the radio equivalent of Dallaire's genocide fax. In El Fashier, Annan meekly sat through a rant. Instead of reading the riot act to the provincial governor and interior minister over support for Janjaweed militias, he said he was happy "that you say that the government accepts that it is a government responsibility to ensure law and order". Traub wonders if "such delicacy had been lost on the audience".

As Adam LeBor points out in Complicity with Evil, caution has become the hallmark of the Annan years. Annan's first public comments on Darfur were issued in December 2003, nine months into the crisis. In his personnel policy, he was too aware of diplomatic niceties to fire corrupt or incompetent officials who have only left office after press revelations. He failed to deal effectively with sexual abuse by peacekeeping missions that tainted the reputation of the UN. His authority was seriously undermined by the Iraq oil-for-food investigation, which concluded that he had shown poor judgement in failing to pursue inquiries against his son, Kojo, and accused him of mismanagement.

LeBor pulls no punches in his indictment of the UN under Annan. With key documents to hand, he rightly identifies the main failings of the system as its lack of accountability, and a cult of neutrality in which "all sides are guilty". He argues that there is no place in the UN for the perpetrators of genocide. So what about the expulsion of Sudan?

Anne Penketh is The Independent's diplomatic editor