There is no rationale which can explain an act of such barbarism against civilians

Liberal credentials count for nothing when seen from the point of view of one furious with certainty

In my own small way, I gave in to the threat of terrorism yesterday. Instead of taking the Tube into the West End, I drove. These days I am spending all hours entombed in an edit suite off the Tottenham Court Road. It means a Tube ride in and out of the city every day. But I had a feeling I haven't experienced since the 11 September attacks. Terror had drifted away from my personal world. Now I find that I am circumscribed once more.

In my own small way, I gave in to the threat of terrorism yesterday. Instead of taking the Tube into the West End, I drove. These days I am spending all hours entombed in an edit suite off the Tottenham Court Road. It means a Tube ride in and out of the city every day. But I had a feeling I haven't experienced since the 11 September attacks. Terror had drifted away from my personal world. Now I find that I am circumscribed once more.

So I drove and on reaching the Marylebone Road I felt like a fool. What were the odds of London being hit the day after Madrid or, indeed, any day? Well, that's the perverse genius of terrorism for you. They, the killers, the unknowable phantoms of the no-warning bomb, hold all the cards. At the time of writing I have no idea whether the Madrid atrocities were carried out by Eta or al-Qa'ida or some previously unknown bunch of killers. I just can't calculate the odds.

The "who did it?" matters, of course, because if it was Eta my feelings of personal fear recede. Basque separatists are not likely to make London their target. The fears don't disappear entirely, but just go back to the abnormally normal low hum of anxiety. I will feel sorrow for the people of Spain whoever the perpetrators. But if by the time you read this the bombings have been shown to be the work of Islamic terrorists, then that awful post-World Trade Centre dread will settle. Because I know what we all know (but trample on in order to get through): it is a matter of time before the bombers will get through our defences.

I lived with the fear of bombing for five years. But in Belfast there were certain rules. I lived there in the latter stages of the Troubles. Most of the time the bombers left a warning. Occasionally they did not or they screwed up the warning. But in the latter stages of the violence there was a certain predictability. When a terrorist group such as the Real IRA shattered those rules at Omagh the political fall-out was immense. The main Republican paramilitary group turned on them.

The new age of terror has no such rules. If the Madrid slaughter is the work of al-Qa'ida then it reinforces what none of us should ever forget. The rule book has been torn up. Even if it is not, we have been given a necessary reminder: we are all targets. We are so used to hearing this from politicians and policemen that we have become blasé. But the threat is real and present. There are men in cities around Europe, the Middle East and Asia who want to do us fatal harm.

Liberal credentials count for nothing faced with the fury of the fundamentalist. In fact, they are contemptible when seen from the point of view of one who is glutted and furious with certainty. The violence of the suicide bombers reduces us all - every last Western one of us - to the status of enemy. As Bin Laden himself has observed: in this war there are no innocent victims.

But what am I guilty of? What are you guilty of? Perhaps we can be accused of indifference to the plight of people in the Middle East, or of actively upholding repressive regimes because we consume their oil. But as the Holocaust survivor Primo Levi has written, the idea of collective guilt is an obscenity. It is intellectually, morally and spiritually wrong. The dead of Madrid do not bear responsibility for what is done in the Occupied Territories or in the torture chambers of Saudi Arabia. If I am blown asunder on the streets of London or harm is done to those I love, whose guilt can be said to have expiated?

Our greatest sin in the eyes of the Wahabi fundamentalist is one of faith. We are guilty of not being like them. We are infidel and thus our lives are much reduced in value. These are the charge sheets of the sectarian bigot desperate to justify his war. They are also the ideas which lie at the root of genocide. Obliterate those who are different until all men are made like me. Create paradise on earth, a world without enemies.

As I say, I'm writing this without any certain knowledge of who carried out the Madrid bombings. But I am certain that someone somewhere will try to rationalise this act. Somebody always does. No matter the horror there is always a justification lurking in some heart somewhere. The apologists may say that the killers were people acting out of rage at the policies of this or that government. Or that they were responding to oppression or had failed to achieve what they wanted through peaceful means. The moral equivalence brigade will say that democratic governments drop bombs on people from great heights. What is the difference between that and planting bombs in public places? Blather. None of it excuses. None of it explains.

When the IRA bombed the Remembrance Day parade in Enniskillen in 1987 they claimed that security forces were the target. Did they know that members of the public would be present at the time? Of course. Did it make a difference? Er. Not really.

Be clear. In war any act which targets civilians - repeat "targets" - is a crime. There are many acts in war where civilians are killed or injured which may or may not be war crimes. The bombing from the air of military targets in areas where civilians are living fall into this category. So to does the bombardment of cities. In Lebanon during the Israeli invasion of 1982 war crimes were undoubtedly committed, so too in Chechnya in the late 1990s and certainly in Vietnam.

For the military forces of nations which are signatories to the international war crimes conventions there are supposed to be punishments for war crimes. More often than not nations get away with the crimes they commit in time of war. But there is at least an international framework which imposes restraint on the behaviour of armies in the field. There is the possibility of legal sanction and this in turn has helped to create a climate where states are far less careless about the taking of innocent human life.

The laws of war and the Geneva Conventions are the most important international legislation we have. Whenever I write in praise of the conventions I get letters from people telling me they are useless. Useless because in the world of al-Qa'ida they mean nothing; useless because they are often ignored by powerful nations. But they do matter. Even as a statement of aspiration - and I believe they are much more than that - the conventions represent our best attempt at defining what seems impossible: a set of rules for warfare.

But there is something more needed, a code that is unwritten but operates as the last restraint on men of war when they are confronted with defenceless civilians. It means that men and women must find in themselves the humanity that separates armed combatant from helpless civilian. The people who bombed Madrid recognised no humanity. Not in themselves and especially not in their victims. The rest of us should remember that. Not out of fear but because someone soon will try to rationalise that butchery. It has no rationale. It is an act without honour.

The writer is a BBC Special Correspondent