Five years ago, perhaps the news that two women had exploded themselves and at least 38 blameless commuters in Moscow would have left us numb, rather than shocked and despairing. We were more used to it then. But half a decade has passed without such an atrocity, and surely everyone had begun to hope the days of brutality were over.
Already Russian officials have speculated about the help the women must have needed to commit these acts of mass murder. Inevitably, they have used the words "outside forces" – that is, a catch-all term that includes anyone they see as enemies of Russia: the CIA, MI6, al-Qaida, and so on. Without such help, they say, Russia's scattered insurgents could surely never have mustered up the strength to do this.
But, in reality, these are some of the cheapest of crimes. All you need to commit mass murder on the Moscow metro is know-how, explosives, wires, detonators – all of which are in plentiful supply in the North Caucasus, almost certainly the source of the attack – and women.
The tragedy of the Chechen war, and the savagery from both sides that has turned Chechnya from a prosperous backwater to a brutalised sink, is that it has created so many traumatised victims. On the Russian side, these are ex-servicemen sent back to their homes with minimal support and memories full of death. On the Chechen side, these are young men scarred by torture, and young women, often raped, often widowed.
In a previous wave of suicide bombings in Moscow, two women blew themselves up at a rock concert in 2003. They had been sent north from Chechnya with a companion, Zarema Muzhakhoyeva, and four days after the murder of the 15 concert-goers, she too set out to kill. Her nerve failed her, however, and she surrendered, becoming the only witness we have to the depths of despair Chechen women have to plumb before they will commit such acts.
She revealed after her arrest how Shamil Basayev, the most terrible of the Chechen warlords until his death in 2006, recruited women to wage war. One of the women who attacked the concert had been widowed; the other ordered to go by her husband. As for her, she had lost her husband, then had her child taken from her. Her attempt to steal back her child had left her in debt and disgrace, and becoming a suicide bomber was the only way she could see of redeeming both.
It is impossible not to condemn someone who sets out to commit slaughter even if, like her, they do not go through with it. But it is also impossible not to condemn a situation where the only support available for despairing young women is the brutal arm of the Chechen resistance.
The reality of the Chechen war is little understood in Russia, where people have willed it to be over for so long they imagine that it is. There was a telling exchange in the trial of Nurpashi Kulayev, who was sentenced to life on terrorism charges as the only survivor of the band that seized the school in Beslan in 2004.
The judge asked Kulayev how he knew so much about military affairs, and the defendant replied: "You have been teaching me this for 10 years". He, like hundreds of thousands of Chechens, had lived in a brutal war for all his adult life and never needed training to tell one kind of tank from another. It is part of the problem that not even a judge knew that.
In 1991, when Chechnya's post-communist leader decided to declare independence, it was not inevitable that the war would turn savage. The other post-Soviet conflicts in the Caucasus – involving Georgia, Azerbaijan and Armenia – were fought between armies of men with guns. Only in Chechnya did schools, theatres and commuter trains become targets.
Every incident has provoked a response; and every response has provoked an incident, until all bonds of humanity have been severed. The first sign this was happening came in 1995, when Basayev and his men seized a hospital to demand an end to the war. His lieutenant tried to justify the attack by saying it was nothing compared to the Russian bombing of his home village.
"Why was the world silent when Shali was bombed, when some 400 people were killed and wounded? In fact, the evil we did in Budyonnovsk was not even 30% of what they did in Shali," he said.
It had appeared that, with Basayev's death, the Chechens had rejected such a terrible, self-defeating philosophy. Now it seems they have not. We just have to hope the Russian response serves to end the cycle of retribution. If they lash out once more, the violence may never end.
Oliver Bullough, who was a Reuters Moscow correspondent, and is now Caucasus Editor for the Institute for War and Peace Reporting.