Leading article: We must not allow this atrocity to undermine our open society

"It is important," declared a visibly rattled Prime Minister from Gleneagles, before returning to London, "that those engaged in terrorism realise that our determination to defend our values and our way of life is greater than their determination to cause death and destruction to innocent people in a desire to impose extremism on the world".

A degree of political hyperbole is understandable on these occasions - although it was noticeable that the Home Secretary, Charles Clarke, projected a more sober and better pitched tone in the Commons yesterday morning. But Tony Blair was undoubtedly right in asserting British determination not to be thrown by such attacks and, in being pictured with the other leaders attending the G8 summit, reminding the world that "each of the countries around the table have some experience of the effects of terrorism". Unlike the Irish campaign, Britain is this time facing a threat that is not peculiar to us nor unprecedented.

Nor, whatever one's feelings about the invasion of Iraq, would it be right for Britain to take its decisions about the future of its troops there on the basis of this attack on its citizens back home. The invasion of Iraq was a mistake. It has helped to radicalise the Middle East and much of the Islamic world against us. But the policy towards that country now cannot be determined by fear of the bomb.

None the less, this is a time for measured consideration as much as high-flown emotion, however awful the casualties or the anger against those who have perpetrated them. The fact is that this was not just an attack designed to hurt, but a relatively sophisticated demonstration of the power of terrorist groups - whether al-Qa'ida or otherwise. In a carefully co-ordinated pattern of explosions both on the Tube and on the bus system, they were able to bring the whole city to a halt and gain maximum publicity at a time when the world's leaders were meeting in Britain, and London was celebrating its Olympic victory. The Prime Minister was forced to leave his meeting to return to London, his fellow leaders of the industrialised nations had to proceed without their chairman, and Parliament had to be convened to discuss the emergency.

It happened despite the constant warnings by security experts that such an assault was in the offing, particularly after the Madrid bombings of March last year, and despite the efforts of the security forces to prevent it. In the event, the preparations of London's emergency services appear to have coped well. Well-laid plans were swung quickly into motion, the Tube was evacuated, bus were journeys were stopped and roads cut off. But it would be entirely wrong either to dismiss the planning behind the terrorist outrage or to underestimate the propaganda coup those behind it sought.

That is not in any way to hand the assassin the "victory" or to pander to his self-estimation. But in seeking to protect ourselves over the future, we must try to evaluate clearly the nature and the likelihood of the threat.

The immediate questions

One immediate question is obviously whether the authorities were right to take quite so long in releasing any information about the incidents or the likely scale of the casualties. They were clearly anxious to avoid panic. But with so phlegmatic a people as the British, and Londoners especially, a policy of openness is often the better course and might actually have prevented some of the traffic chaos and civilian confusion that reigned for several hours in the nation's capital. Nor did the financial centre, which had behaved so well when the IRA bombed the Stock Exchange, cover itself in glory this time, rushing to sell off shares and close bank branches. There is a careful balance to be struck between due caution in the face of such atrocities and ensuring that life proceeds normally. The authorities need to look in due time at this incident to see whether the right balance was eventually struck.

In a deeper sense, however, we must try to understand better the forces we are up against. Categorising the situation as a "war against terror", or talking in terms of facing an attack on democracy and "what we hold dear in this country and in other civilised nations throughout the world" (to use the Prime Minister's words) is tempting but unhelpful. Our invasion of Iraq took the war to the Middle East, not the other way round.

Still less is it useful to use such terms as an excuse for introducing ever more restrictive controls on immigration or the freedoms of ours own citizens. ID cards did not prevent the Madrid bombings, and it has yet to be proved that they would have done so in London's case. Locking up suspects without the right of a trial and placing them under detention or control for indefinite periods does nothing to improve our security and much to antagonise individuals and communities.

The terrorist threat comes obviously from international terror organisations - although it is far from clear that al-Qa'ida is anything like the centralised force it is often depicted as - but also from home-grown and quite localised groups who may or may not be drawn to violence. One of the most important questions that the police and security forces will have to determine in the coming months is whether these attacks were from international or national sources.

The right response

Yet in doing so, Britain must be careful not to radicalise groups here or drive its own citizens to sympathise with extremists by seeming to blame whole sections of society, and to bear down on them. The primary aim of extremists in these attacks is not to undermine British society as such or to "cow us", as Tony Blair would have it. It is to demonstrate across the news channels of the world, and Arab TV in particular, that they have the capacity to bring one of the world's great cities to a halt and to "punish" Britain for its invasion of Iraq. Against the might of Western arms, they wish to show their brethren that they have the means and ability to strike back. The secondary aim is to produce a repressive reaction that will produce new converts to the cause.

The most effective way of dealing with this threat is through prevention, by first-class intelligence and dogged police work. The worst response is to play into the hands of the terrorist. London won its Olympics bid on a pitch that emphasised its multiculturalism, its tolerance and its openness to the outside world. We must not lose this as we react to this outrage, however abominable it is.