It is customary to talk about the “lessons” of genocide — in the Nazi extermination camps, in the Khmer Rouge killing fields of Cambodia, in the Japanese massacre of 300,000 Chinese in Nanking, in the Serbian ethnic cleansing of Srebrenica. This week, the “lesson” is from Rwanda, where ten years ago Hutus turned on Tutsis and began the slaughter 800,000 of them in a savage 100-day orgy of violence.

But what is it that most of us can learn from such “lessons” except that at the start of the 21st century, when man is reaching ever further into space, when the Internet has revolutionized communications and researchers are decoding the genetic signals of life itself, people are still capable of descending into barbarity.

The racist argument that Rwanda is an ill-educated and primitive society does not wash. Both Germans and Serbians were advanced and apparently civilized people, yet from their midst there stepped forward large numbers of individuals prepared to perpetrate these enormities. Though their cultures may be miles apart, there is no substantive difference between a Hutu member of the Interahamwe killing squads, waving his bloodstained machete, and a middle-class member of a German SS battalion who was prepared to shoot hundreds of Polish Jews in the back of the head as part of a day’s work.

Can any minds so warped by racial hatred, so stained with prejudice learn anything at all from the atrocities perpetrated in the past by other deluded people? Will the knowledge that those responsible for these crimes have often been punished, stop future genocidal massacres on such a horrific scale? It would be rash to believe that the lessons of the past will defuse future hatred. Terrible though it is to admit, there will be more Rwandas, more Dachaus, more Srebrenicas.

But there is one lesson that can be learned by all decent people who recoil at such cruelty. It is that history has shown time and again that the outside world has stood by for far too long before intervening in these ethnic bloodbaths. Governments have generally preferred to avoid becoming involved until media revelations of the horrors have forced a public outcry for action. The Allied governments became aware of Nazi genocide very soon after it began but chose to keep quiet. Thus its full extent did not become clear until the death camps were liberated by Allied troops in the closing stages of the war, by which time for millions of Jews, Gypsies and others it was too late.

At a personal level, the lesson is surely that horrors which are unimaginable for all sane and right-thinking people have already happened in our lifetime and will probably happen again. We cannot again shut our own eyes in disbelief and do nothing.