The fluttering blue flag of the United Nations is meant to be a reassuring presence - law and rights should hold sway where it flies. But that flag has lost some of its allure in recent years. One reason is its failure - and that of the "international community" in general - to stop the most serious of crimes: genocide.

The evidence of failure is incontrovertible. The precise numbers will never be known, but perhaps 200,000 died in Bosnia, 800,000 in Rwanda and now 300,000 have died - are dying - in Darfur. The question is how this can happen when the "international community", as embodied in the UN, has promised to stop it. The genocide convention of 1948 commits signatories to intervention. More recently, almost all states have signed up to the "responsibility to protect" civilians, even to the extent of intervening in other countries. And yet the killing goes on.

Carne Ross is a former British diplomat and now director of the non-profit diplomatic advisory group Independent Diplomat

Adam LeBor's new book Complicity with Evil - The UN in the Age of Modern Genocide cuts through the usual waffle and shirking of responsibility officials offer to excuse their gross inadequacy. In a driving and angry tale, he tells the story of each of these three mass murders: who did what to whom, and who failed in their duty to stop them. Some of his material is familiar from other sources, but his retelling is still shocking. Much is fresher, with first-person accounts of these very contemporary horrors. And with mass murder happening right now - today - citizens, and our governments, still need to be reminded of our obligations.

Each episode is told as straightforward narrative with phone calls, telegrams, diplomatic meetings recounted in a depressing litany of activity without effect. Most revolve around the UN Security Council, the body charged with guaranteeing the world's peace and security. I spent over four years working there for the United Kingdom delegation, and rare is an account of that place that so accurately conveys its strange mixture of high politics, the low prejudices of the ambassadors and officials who work there, and its utter detachment from the reality it is claiming to arbitrate on the world's behalf.

We see the British ambassador expertly defining "safe areas" in Bosnia without defining the small detail of how they are to be kept safe. We watch the UN special envoy argue against air-strikes as Serb general Ratko Mladic's tanks roll unopposed into one such safe area, Srebrenica, the prelude to a disgusting bout of murder and violation. Mladic has at least been indicted for his crimes, though he is still free. But what of that special envoy (or that ambassador), LeBor demands; where is the accountability for their actions?

In Rwanda, we read of the so-called "genocide fax" from General Dallaire, commander of the UN "peacekeeping" force. Based on clear intelligence from a source Dallaire thought was reliable - indeed a future genocidaire - this communication warned of imminent plans for a killing spree by Hutu militias. Kofi Annan, then head of UN peacekeeping, failed to pass on the fax to the UN Security Council. When the genocide began, Belgium, the main troop provider, persuaded the other council members to withdraw all the peacekeepers, leaving the Tutsis and moderate Hutus to their deaths. Playing weasel words over the definition of genocide, the United States, Britain and others shirked their legal - not to speak of moral - responsibility to act.

In Darfur, the story is more complicated and inevitably, as it is more recent and indeed current, less well-documented. LeBor claims that the UN has failed promptly to inform the Security Council, and thus the world, of the genocide. But his own account tells how some brave UN officials did just that. Without offering enough evidence, he argues that the UN avoided raising alarms over Darfur in order to preserve the fragile north-south Sudan peace deal, which had ended twenty years of war that may itself have cost two million lives.

Whatever the precise truth, the result is dismally familiar from his earlier examples. Thanks largely to Russian and Chinese support for Khartoum, the Security Council has failed to impose the UN force necessary to protect civilians. Meanwhile, the US maintains an ambivalent - some would say hypocritical - relationship with a Sudanese government that offers cooperation in the "global war on terror" while abetting the massacre of its own citizens.

Whose responsibility?

The conventional way of talking about such episodes is to attribute motives and actions to states and institutions ("the UK believed that Nato intervention would exacerbate the conflict"). But where LeBor's account differs is that he takes the analysis - and indeed the blame - a step further. In each case, there were individuals both in governments and the UN who should, at crucial moments, have helped, and who failed to do so. Some will be discomfited by it (not least those accused in this account), but I welcome this ad hominem approach. For too long anonymous diplomats, soldiers and officials have hidden behind the name of their state or institution. LeBor shows convincingly that individual actions at crucial moments make a difference - for good or, more often in these cases, ill.

The book's introduction suggests that it will prove the UN's complicity with genocide under the legal concept of "command responsibility". This charge is more commonly applied to Balkan warlords who fail to prevent war crimes by their troops. It is a very provocative accusation. While there are individuals who have much to be ashamed of in their conduct, I am unconvinced that this amounts to criminal guilt.

Troubling too is that it is not clear that everyone LeBor accuses has had the chance to give their side of the story. His book, moreover, itself shows that there is good as well as incompetence in the UN, just as there are states which seek to do the right thing while others look away, or even condone. Hence my reservation about its overstated title, a title which I fear will increase its appeal to those, especially in the United States, who love to bash the United Nations - and with it the notion of international rules.

Life is complicated for the United Nations. Its officials cannot afford to offend the powerful countries who dominate the Security Council. If they do, they are quickly stamped upon - I have seen it happen. But too often, UN officials try to be even-handed when blame should be clear, and sometimes even bend the facts to suit the diplomacy. The necessary subtlety of the UN's role should not excuse failure to speak the truth, especially when that truth is mass murder.