First it was disbelief, then shock and now sadness. The world has been profoundly moved by the attack on New York and Washington, but why have these deaths apparently meant more than those killed by both sides in the Middle East this year, Serbia two years ago and the deaths that presumably will come in the months and years ahead as America wages the first war of the 21st century?

The attack on the World Trade Centre looms small in terms of loss of life compared with natural disasters, or the 20th century's wars, or modern famines, both inevitable and man-made. It pales against the Blitz, the fire-bombing of Dresden, the siege of Leningrad, the razing of Berlin, atomic bombing of Hiroshima and Nagasaki. And yet it is already one of the defining events of this new century.

Nor is it as though America is not a violent country. It is one of the most dangerous on Earth. No affluent country matches its rates of homicide. No First World country has guns in the hands of so many children.

Its entertainment culture is saturated with violence, horror and cruelty. It has even created a genre in urban holocaust. The cultural critic Mark Davis estimates that Los Angeles was destroyed in celluloid or print no fewer than 138 times between 1909 and 1996.

And yet it matters not how many times we have rehearsed disaster: the shock is that this tragedy happened in a time and place that was meant to be immune.

It was a beautiful autumn morning in New York, after what has been an unpleasant summer. The city has become safer than it has been for decades. We have now read and heard many stories of peaceful, ordinary people going about their morning when this disaster exploded into their lives.

And amid the tragedy of the loss of life, what has also been moving has been the quiet courage, the steadiness under fire, the mutual help and support, the absolute decency of everyone. The people who have borne the suffering inflicted on them for crimes that are not of their doing, have been utterly admirable.

Even so, there is a difference in their trauma, though none of us who have lived safe lives in an orderly country can criticise those who still feel some bitterness that this horrible crime is little different from what America or its allies have inflicted on them in Serbia, or Gaza.

But there are differences in the American trauma. First, it has revealed the vulnerability of modern cities, not just to terrorism, but to war. Could any modern city continue to function and resist as did London during the Blitz now that we are so dependent on technology and our buildings so big?

Modern cities are probably now indefensible and their civilian populations helpless. To use the jargon of one of the many experts interviewed this past week: Manhattan is "target rich", whereas Afghanistan is "target poor". A couple of strategic strikes and you can achieve what might take months in Afghanistan.

The second difference is that this has happened to the people to whom such things were not meant to happen. And what has been shattered has been any First World illusions of immunity to war and terrorism. Even our modern wars have been fought by remote control, or at least not in our streets and fields.

In the affluent West we have become obsessed with the control of risk. If tragedy does breach our defences, then monetary compensation will redress our losses. Money cannot bring back loved ones, but we do put a money value on suffering and material loss.

The insurance industry has been one of the most important players in modern capitalism. Since the 18th century, it has accumulated more capital than any other part of the global economy.

And when natural disasters, climate change and now terrorism hold us to ransom, the insurance industry holds the world to ransom.

Thus it may well be the insurance industry that determines whether this crisis will bring on a global economic collapse. In other words, the economy and the airlines can only afford to resume normal activity if the insurance companies can afford to indemnify them. And if they can't, as they are currently saying, then either we take the risk or we stop "doing".

For those who have been personally traumatised by this tragedy, it will be hard to resume their everyday lives. It is for their sake, as well as our own, that the rest of us who only stood by and watched must learn to live with risk and, perhaps because of that, accept more readily the accountability that goes with wealth and power.

Janet McCalman teaches history of health and medicine at Melbourne University.

E-mail: janetsm@unimelb.edu.au