At 4pm Central Time two weeks ago today, the US national weather service warned that the hurricane about to hit New Orleans would cause 'human suffering incredible by modern standards'. The following morning, as citizens drowned on their front porches, President George W Bush attended a cake-cutting photo-opportunity to mark Senator John McCain's 69th birthday.

And now another anniversary rolls around. On 11 September 2001, the world reached out to a stricken America, unaware that US helplessness was a brief prelude to an era of power run mad. From Manhattan to Mesopotamia to the Mississippi, we have come full circle. Hurricane Katrina has turned a superpower into a victim again.

Only this time, few rally to the President. Rotting bodies lie like rubbish sacks in the streets of New Orleans, 42 per cent of Americans think Bush has done a 'bad' or 'terrible' job, and the world looks at his catastrophe management in horror and derision.

Blaming Bush is easy. That is not to say it is wrong. It would have been better if scientists' warnings had been heeded and the levees strengthened. It would have been preferable if America had not housed its poorest below the Plimsoll line of civilisation, and if the emergency controller, now sidelined, had not been a klutz called 'Brownie' who used to run gymkhanas. Ideally, the President would learn to prioritise national disasters and birthday parties.

But the hurricane is not about one scapegoat, however powerful, nor is it just a fable of America. Britons may be shocked that obese inhabitants of the richest nation can be victims of starvation too. They may be startled that many New Orleans citizens are poor black people who might shoot and loot and who have never set foot in the French quarter, with its filigree balconies, jazz clubs and a population that is 95 per cent white. But Britain, with its own gaping social divide, has no excuse for smugness.

The universal lesson of Katrina is broader, though, than inequality. Timothy Garton Ash argues that we have witnessed how, with the removal of the staples of civilised life, such as water and personal security, people revert to a 'Hobbesian state of nature, a war of all against all'. At the hint of a dirty bomb or some other apocalyptic onslaught, societies could become 'decivilised'.

This is a very gloomy creed. It is true that, in extremis, hungry flood victims are rarely to be mistaken for Jane Austen heroines. You will not find them sitting around debating whether Hobbes's gospel of state control was more relevant than the benign anarchy outlined by Proudhon, who thought that 'property was theft'. They will get down to Wal-Mart and grab some food and drink, if necessary at rifle point. Such action may or may not seem more despicable than the online looting of white-collar barbarians now setting up bogus flood relief appeals.

The Lord of the Flies scenario is, by definition, nothing new. The Greeks coined the word 'barbaros' for anyone who did not speak their language. To Marx, barbarism was a synonym for capitalist destruction, a theme embroidered by Osama bin Laden's favourite writer, Sayyid Qutb, who used the term jahiliyya to label the West a materialist hell meriting attack. And so to 9/11 and a new chapter in barbaric action. The timeline linking September 2001 to today is studded with the headstones of its antagonists and martyrs.

They include as many as 27,000 Iraqi citizens; the 2,000 dead US and British soldiers; the massacred of Bali and Istanbul, Riyadh, Madrid, Casablanca, London. There are the individuals whose histories dominated the narrative for a while; Margaret Hassan, Kenneth Bigley, David Kelly: the aid worker, the contractor, the government inspector.

Western pugilism cannot justify the outrages of jihadism. But few, bar the US President and the British Prime Minister, would seriously argue that an unlawful Iraq war, fought against world opinion, has not fomented terrorism and laid innocent people everywhere open to attack and peril. North Korea has the bomb and Iran may not be far behind. Bush, and now Blair, have defied international rules to welcome India to their nuclear club.

If the people who strolled to work in the Twin Towers on a cloudless September morning could see the blood that saturates their graves, they would be incredulous. Many would also be appalled, not only by fanatics who want to rewind the world to a 7th-century caliphate but by the forward planners of the West.

On decivilisation, we do not have much to learn from a few hungry looters. Those interested in the seeds of savagery should look elsewhere. The destitute driven to anarchy are simply victims. Those who commit grave crimes under the cover of disaster deserve punishment by the law, but even they do not undermine civilisation in the same way as people who wear suits, kiss babies and sign the orders that enable, though never command, the outrages at Abu Ghraib, Guantanamo and Camp Breadbasket. Crowds may look menacing, but the powerful are much better at torture and havoc than the weak.

For Americans, and all other nations, today is the anniversary of year zero. The resurgence of religion in a world reforged four years ago meshed with Bush's war of good versus evil; of right-wing Christendom versus radical Islam. The Lord's carelessness in allowing 9/11 to happen might deepen hardline faith, but that was murder in the second degree. His callousness in unleashing an act of God has shaken Bible Belt America to its pious roots.

That doubt may do some good, if a side-effect of Katrina is to downplay the hand of the Almighty in US politics. Already Bush is proving much less eager than many feared to cram the Supreme Court with anti-abortion zealots. His country might learn to love the gun a little less and the poor a little more. There are more lessons, as relevant for Britain as for America.

The solidarity instilled by 9/11 made electorates too credulous. Even the most vehement opponents of invading Iraq were shocked to discover that the evidence for Saddam's weapons of mass destruction was no more than the fantasy of leaders spoiling for a war.

There will be other crusades. Bush still menaces Iran. Blair talks of justice and human rights as if they are the shield and armoury of jihadis, rather than the pillars of civilisation. The discarded dead of New Orleans offer another story: of social division, craven politicians and incompetent public servants.

But, most of all, the disaster has been a reminder to all who hold the powerful to account. Tragedy once conferred upon our leaders a wisdom they did not possess and a trust they did not merit. Never again.

mary.riddell@observer.co.uk