But al-Qaeda is less a hierarchical organisation out of James Bond led by a sinister mastermind, than a dynamic dialogue between like-minded radicals conducted via mosques, radical publications and the internet. A specific order is almost redundant as individual groups know exactly what must be done and when, adapting themselves to new security constraints and to new targets.

And what appears to have been 'understood' before the attack on Mombasa was that it was the right time to polarise the war on terrorism. Just at the moment Bush and his allies had constructed a grudging consent from the Arab world for its tough line on Iraq, Jihad International brought in Israel. In his belligerent threat to hunt down the perpetrators of the Mombasa attacks, Israel's right-wing prime minister Ariel Sharon has threatened to upset the delicate consensus between America and its allies on the issue of the war on terrorism, and on the Security Council over Iraq.

What is more terrifying still is the notion among the West's political classes that it is an organisation, not an idea, that they are fighting. With each new arrest, each new targeted killing, we congratulate ourselves that we are winning - until the next atrocity takes place. All the while, we fail to tackle the ideas that replace each arrested or dead terrorist with a new recruit.

As the tens of millions dead in the last century demonstrated, ideas - no matter that they are venal ones like Nazism or Stalinism - can be as hard to kill as they are lethally and stupidly persistent. But in a war of ideas, to do nothing is the worst of all options.