In 1994 an estimated one million people were killed in Rwanda in a deliberate, public, and political campaign. The murders were directed by central government. The army was used to drive people from their homes and assemble them at places of slaughter. A militia of the unemployed was trained to kill 1000 people every 20 minutes. Local administrators organised the disposal of bodies in rubbish trucks. The slaughter continued unhindered for three months. Many victims bled to death, mutilated with the simplest of agricultural tools - the machete - and yet the rate of extermination was five times faster than that of the Nazis.

The killing in Rwanda was portrayed as a tribal battle between the Hutu and Tutsi, a senseless civil war caused by old rivalry. The true facts are very different. The intention of those who planned this slaughter was to eliminate the Tutsi. This was genocide. Under international law, nothing is graver in a criminal sense than a state policy to eliminate a human group. It is the ultimate crime.

A key element in the act of genocide is an ideology, based on racial grounds, which serves to legitimise any act, no matter how horrendous. So it was in Rwanda. The Tutsi were demonised, and defined as being outside human existence - vermin and subhuman.

Philip Gourevitch's book is a series of stories told by the survivors of this terrible event, as he travelled the country gathering the human experiences which lie behind a genocide. There is plenty of material for a journalist in Rwanda, and these are stories to break the heart. Here is a mother, trying to escape with her children, and hiding out in the marshes by the river, when she hears her daughter crying out: ''No, don't kill us, we have money. I have money, don't kill me,'' and she realises that her children have been caught.

Gourevitch has interviewed people across the divide - those glad to have cheated death, and others who wished they had died. He has met perpetrators of genocide who show no sign of remorse and includes those vital stories of bravery, of Hutu who risked their lives for Tutsi. And he meets killers, those who still believe in the rectitude of their behaviour.

Gourevitch is a feature writer on the staff of the New Yorker magazine, and he follows a tradition of powerful reportage, using a style in which the journalist is part of the narrative. There is a scene, near the opening of the book, in which Gourevitch describes his first visit to a massacre site. He writes: ''The skeleton is a beautiful thing. The randomness of the fallen forms, the strange tranquillity of their rude exposure, the skull here, the arm bent in some uninterpretable gesture there . . .''

Gourevitch's great skill is to tell the history of this tiny and tormented country through the Rwandan people he befriends, and so clearly that it is soon obvious to the reader how the fascist ideology of Hutu Power came to have such tremendous influence. The genocide was the product of authoritarianism, of decades of political theorising and indoctrination. Rwanda was one of the most meticulously administered states on earth, whether under the rule of the Tutsi monarchy, or (after the revolution of 1959) under the control of the Hutu. The race divide may have been artificial, but Rwanda still managed to go from one racial dictatorship to another. In Rwanda, the killing of innocents became a political tradition. The reason why the speed of the slaughter in 1994 broke the world's most atrocious records was because so many people joined in. As Gourevitch points out, this arrangement eliminated

any question of accountability which might arise; if everyone was implicated, implication became meaningless. Neighbour killed neighbour in a country of conformity and blind obedience.

A large part of the Tutsi population thought their extermination was inevitable, and Gourevitch's title is taken from a letter written just after the genocide began in April 1994, when seven adventist priests, among the thousands of people gathered in a church, wrote a chilling letter to the church president that they wished to inform him that tomorrow they would be killed with their families.

None of these stories should be lost, and Gourevitch's book will be treasured by Rwandans, for it is their history.

But the Rwanda genocide is also part of our own history; it has proved how far we have come from even beginning to understand the promise that the Holocaust must never happen again. The promise was enshrined in the 1948 Convention on the Prevention and Punishment of the Crime of Genocide, the world's first human rights treaty, the first truly universal, comprehensive, and codified protection of human rights. The genocide convention is of unique and symbolic importance, for it stands for a fundamental and important principle: that whatever evil may befall any group or nation or people, it is a matter of concern not just for that group but for the entire human family.

To prevent a repetition of genocide, the convention relies on the United Nations, on its procedures and institutions, now shown to be incapable of fulfiling this task, even when the evidence is indisputable, as in Rwanda. It is a harsh truth, but if we cannot prevent genocide in Rwanda then we cannot prevent it anywhere. In Rwanda a genocide was planned and executed in a country in which the whole of the international community was intimately involved.

Gourevitch lays the blame on the Clinton administration, but devotes several pages to events in the UN Secretariat, giving an impression that crucial to the failure over Rwanda was the fact that UN bureaucrats did not react to a specific warning that genocide was being prepared. In fact, everyone knew it was coming. One peacekeeper said that in Rwanda in the weeks beforehand, the genocide ''hung in the air''.

Gourevitch admonishes the peacekeepers in UNAMIR (UN Assistance Mission in Rwanda), blaming them for inaction. He writes that while these peacekeepers were capable of shooting dogs, on a feeding frenzy of human flesh and bone, these same soldiers were quite incapable of shooting people committing genocide. He writes: ''After months, during which Rwandans had been left to wonder whether UN troops knew how to shoot, because they never used their excellent weapons to stop the extermination of civilians, it turned out the peacekeepers were very good shots.''

No detail here about the agonising choices which their force commander had to make, his lack of the most basic equipment, and his own abandonment by the Security Council. Nor does this book explain to the reader that some 400 UN peacekeepers stayed on in Rwanda, even after the UN pulled out its forces - and that they were volunteers. While diplomats and politicians were arguing that nothing could be done, this small group was saving an estimated 25,000 people. The International Committee of the Red Cross was in Rwanda too, picking up the debris of genocide. These efforts were a drop of humanity in an ocean of blood, but it is not true to say the whole world abandoned Rwanda.

In many ways Gourevitch is uninformed about the most important issues raised by this horrifying event. The genocide was a great crime, and it deserves precise documentation. This book makes a first attempt. But a million questions remain about the exact circumstances of all of this, the shady machinations behind the scenes which helped to facilitate genocide, and the international community's litany of errors leading finally to catastrophe.

n Linda Melvern is the author of The Ultimate Crime: Who Betrayed the UN and Why