The chaos in the UK's airports yesterday, caused by exhaustive efforts to screen and sift hand baggage, was a small victory for terrorists. The murder of hundreds of travellers would have been an incomparably larger one. That is the nature of the balancing act that we entrust to our government.

The immediate question will be whether the security clampdown prevented an attack. The police seem confident, but the truth is likely to emerge only over time. The police and intelligence services have not been discouraged by earlier false alarms. In June they launched a vast security operation in east London, based on intelligence reports. They shot a man but found no sign of the chemical weapons they had expected. Last year police shot and killed a Brazilian electrician, Jean Charles de Menezes, whom they had mistaken for a suicide bomber. Yet they also failed to anticipate the July 7 bombings last year, in which 52 people were murdered.