Deterrence in cyber-warfare is more uncertain than, say, in nuclear strategy: there is no mutually assured destruction, the dividing line between criminality and war is blurred and identifying attacking computers, let alone the fingers on the keyboards, is difficult. Retaliation need not be confined to cyberspace; the one system that is certainly not linked to the public internet is America’s nuclear firing chain. Still, the more likely use of cyber-weapons is probably not to bring about electronic apocalypse, but as tools of limited warfare.

Cyber-weapons are most effective in the hands of big states. But because they are cheap, they may be most useful to the comparatively weak. They may well suit terrorists. Fortunately, perhaps, the likes of al-Qaeda have mostly used the internet for propaganda and communication. It may be that jihadists lack the ability to, say, induce a refinery to blow itself up. Or it may be that they prefer the gory theatre of suicide-bombings to the anonymity of computer sabotage—for now.