A day of infamy, somebody said, recalling the previous direct attack on the United States, at Pearl Harbour nearly 60 years before. But the worldwide impact of September 11, 2001, was much greater than any conventional air raid. It bears a better comparison with Hiroshima. In the same way that the atomic bomb seared the latter part of the 20th century, the terrorism that destroyed the twin towers will haunt national politics and international affairs for the foreseeable future.

Hiroshima and Nagasaki were obliterated by a new and terrifying technology, a weapon of mass destruction. New York and Washington were hit last year by old and familiar technology, airliners, commandeered with weapons of everyday simplicity.

The centres of economic and military power in the modern world proved vulnerable to box-cutters. That is the meaning of September 11. Societies such as ours suddenly realised how vulnerable they really are, and how easily their equipment and services can be turned against them.

Much attention has been given since September 11 to the security of airliners, and it needed that attention. But there are innumerable other elements of modern life that could be converted to deadly purposes by those with a mind to do so. Soon after September 11, anthrax was sent through the US mail.

Nothing is to be gained by suggesting other network services that could be easily manipulated in a malign cause. They will be all too obvious to any group with a grudge of sufficient proportions. For all the discussion of the lessons of September 11, there is not much sign that those in charge of all kinds of public services are trying to anticipate the uses terrorists might make of them and taking steps to prevent them.

A year on, the lesson of vulnerability that September 11 taught has been assuaged somewhat by retaliation in Afghanistan. The US and many allies have removed the Taleban, destroyed the training camps of the al Qaeda organisation and, hopefully, its wider network. But its leaders, with one exception, have not been captured or even located. One or two incidents in the Middle East this year suggest they are still capable of organising death and destruction.

Hundreds of their followers have been captured at the battles in Afghanistan and have been detained without trial now for a disturbingly long time. The US launched its retaliatory campaign in the name of law, and it needs to demonstrate that principle in its treatment of enemies. US dominance of the modern world is being compared to that of Rome in ancient times. It is much more than the leading military power that Britain was before it, and Spain a few centuries earlier. America, like Rome, is the predominant cultural, commercial and political power. It promotes a way of life and values that rest on democracy and law.

The US was not devastated by the attack from within, merely awakened. The country realised that distant conflicts can reach across the oceans and hit Americans at home. It had not happened on the US mainland for a century. The realisation may turn out to have cured the country of its isolationist instinct at long last. Americans might no longer need much convincing that their Government should try to help to resolve foreign disputes, and risk American lives if necessary. American security can depend on it.

We thought the 20th century was the American century but for the most part the US was a reluctant power, committed only to the Cold War with communism. Thanks to the events a year ago today, the country that ultimately provides our security is alert and determined to play the world's guardian.

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