It will remain forever a defining moment of the 21st century, a sentinel marker in an age when so much is quickly forgotten. This was the point at which the vulnerability of the United States of America - economically and militarily the most powerful nation in the world - was exposed. The opening sentence of the subsequent official commission of inquiry captures it precisely: "At 8.46 on the morning of September 11, 2001, the United States became a nation transformed." It was not nuclear weapons or an invading army, but a handful of ideologically warped terrorists engaged in a cowardly war, using means that even the most powerful nation in the world could not defend itself against. At 8.46am, a jetliner carrying thousands of litres of fuel slammed into one of the World Trade Centre twin towers in New York. Seventeen minutes later, a second plane flew into the second tower. Two more aircraft crashed that morning: one into thea Pentagon, another into a field after passengers overcame hijackers, armed with the knowledge that their nation was under attack. The huge twin towers burned then collapsed. Almost 3000 people died directly as a result of the attacks. Four years later, footage showing the first and second planes ploughing into the iconic glass-and-metal towers is no less chilling. These are events etched deep in the global consciousness of the information age.

There had been warnings of catastrophe, notably the bombing of the USS Cole in Yemen in 2000 and intelligence suggesting al Qaeda was preparing an attack of some sort in the US. Despite the prodigious intelligence-gathering of the US government community, no one had joined the do6ts. After the attacks came introspection. The 585-page report of the National Commission on Terrorist Attacks Upon the United States (better known as the 9/11 Commission) was released last year. It found that no one person or agency was to blame. It recommended refocusing national efforts on combating terrorism in a manner that centralised power and intelligence even further. The report drew praise from the Government but was widely criticised by others, one lcommentator concluding it "offers peace through exculpation, evasion, and entertainment and in doing so dangerously re-energises a national relish for fantasy".

The US engaged in a full-on war on terror that saw it engage and topple the government of Afghanistan, understood to have given succour to the leaders and foot soldiers of al-Qaeda, and then embarked upon a bloody and unfinished war in Iraq, named by US President George Bush as part of an "axis of evil". Australia and Britain have remained staunch allies in this war without borders or battlefields. Much of the rest of the international community, notably the European Union, is wary of the US approach. There have been some dividends. Libya appears to have relinquished its past as an enclave of terrorism, and there has been a push to reform and/ reinvigorate the United Nations.

Australians discovered they were not too remote from terrorism when bombers struck in the popular Indonesian resort island of Bali in October 2002. Of the 202 killed, 88 were Australians. The bombing attack on the Australian embassy in Jakarta one year ago underlined the fact that Australians must be considered targets of terrorist attack. Other attacks in Jakarta, Madrid and most recently in London have shown the struggle against this modern scourge is far from ended. There have been other debits. The climate of suspicion fostered since the September 11 attacks has, almost inevitablyc, led to an erosion of some freedoms.

So four years on from September 11, in a world that is fraught with dangers, what has the superpower learned? The answer to that conundrum may well lie much closer to home. When hurricane Katrina lashed the Gulf states a fortnight ago, America was exposed as vulnerable again. The death toll from this natural disaster will probably far exceed1 that of the September 11 attacks. But it is the Federal Government's response - or more precisely - its shambolic and hopelessly delayed response to Hurricane Katrina, that has caused Americans again to become introspective. Many Americans have been embarrassed and shocked by the depth of the economic and racial divide that has emerged from the chaos of New Orleans and the other affected communities. For a few crucial days in the immediate aftermath of the hurricane, far short of saving the world from tyranny, America looked as if it could not care for its own. The National Guard was diminished by the demands of the war in Iraq. Ironically, the Federal Emergency Management Authority - the body charged with the task of managing such crises - had been integrated into the terrorism-focused Department of Homeland Security. The loss of many disillusioned senior FEMA staff arguably left it ill-equipped to cope with the demands of the present crisis. Yesterday, the White House lost faith in the head of FEMA, Michael Brown, effectively sacking him.

Americans will determine whether the billions of dollars spent on the war against terrorism at the expense of other programs have been well spent. But on this day in particular, it is worth remembering how swiftly and irreversibly the world changed on that terrifying day in September 2001, and how America used its economic and political leadership to decisively respond to that event. Had it not, how much more vulnerable to terrorist attack would the world be today? That is almost impossible to answer.