By Paul Reynolds
World Affairs correspondent, BBC News website
Hurricane Katrina will be as big a test for the United States as was the attack on the World Trade Center in 2001.
It will test not just the leadership of President Bush but the whole country.
New Orleans under water: it will be rebuilt
The crisis raises widely different issues.
Should civilisation be planted in areas where nature has declared its hostility? Does a society hold together or split apart in a disaster?
How does it marshal its resources for recovery? How did it act in advance to try to prevent or minimise the danger? How did it react after the event?
Having lived in America on and off for seven years and having seen its can-do attitude when faced with a natural disaster, I am in no doubt that in due course (and it will be long course), New Orleans and the other Gulf communities will be recovered and rebuilt.
That has happened before, in San Francisco after the 1906 earthquake for example. It will happen again.
Update on Thursday evening London time: One has to say that reports indicate that the intitial response has not been as fast as might have been expected. Thousands of people are stranded in the city and are getting angry and desperate. My prediction is that the evacuation will pick up speed but the authorities did have several days in which to plan for this and the delay is notable.
These are some of the wider considerations:
Mr Bush returned to Washington from his holiday in Texas and made a speech whose rhetoric, unlike his first effort on 11 September 2001, did rise to the occasion.
"We are dealing with one of the worst natural disasters in our nation's history," he said.
But this is only the start. Strong words imply strong actions.
Even his supporters like columnist Peggy Noonan are concerned that the federal government was a bit slow off the mark. She wrote in the Wall Street Journal: "More was needed in terms of sending a US military presence into New Orleans."
She asked about Mr Bush: "Does he understand that what has happened in our Gulf is as important as what is happening in the other Gulf?"
Mr Bush has acted swiftly on the energy front by releasing some of the US strategic petroleum reserve. The signs are that the White House does not intend to be behind the curve.
The disaster is already being politicised.
The New York Times says that Mr Bush's foreign policy is linked to the issue of how he manages these monumental problems.
"He saw up close the political damage done to his father 13 years ago this week, when the senior Mr Bush was dispatching fighter jets to maintain a no-fly zone over parts of Iraq and promoting his trade agenda while 250,000 Floridians were reeling from the impact of Hurricane Andrew," the paper says.
Mr Bush believes that America can both stabilise Iraq and recover New Orleans and the other lost cities.
He will not be changed over Iraq. In fact, the aftermath of the hurricane led to a strong speech on Iraq being neglected.
In it he upped his rhetoric. "I've made my decision: we will stay on the offensive," he said. "We will stand with the people of Iraq, and we will prevail."
Could it have been prevented?
The New Orleans Times-Picayune, its printing presses under water, and operating online only, said: "No-one can say they didn't see it coming.
"Now in the wake of one of the worst storms ever, serious questions are being asked about the lack of preparation."
The looters and the left-behind: what next for them?
That newspaper had previously written a series of articles about whether federal funding for flood protection was enough.
Already, Mr Bush's political enemies have begun to circle him on this issue.
A former official in the Clinton administration, Sydney Blumenthal, has written in Der Spiegel: "In early 2001, the Federal Emergency Management Agency issued a report stating that a hurricane striking New Orleans was one of the three most likely disasters in the US, including a terrorist attack on New York City.
"But by 2003 the federal funding for the flood control project essentially dried up as it was drained into the Iraq war."
It remains to be seen whether any extra funding would have made any difference. For example, would the money have come through in time and would it have been spent on those places which gave way?
The fabric of society
The Mayor of New Orleans made a good call in ordering an evacuation of the city in advance. He offered public transport. But not everyone listens. So not everyone left.
Update on Thursday evening London time: There has been some dispute about why so many were left behind or stayed behind. In Cuba they would have been taken out. Whether enough transport was available or whether people simply ignored the warnings will have to be examined for lessons to be learned in future.
The fact remains that It is noticeable on television that those left behind or stayed behind appear to be poor African-Americans.
Some have been looting. Some loot to get food for their families in the absence of food from elsewhere. Some loot for gain. Many have just been waiting.
The fate of these people will help determine the overall effectiveness of the relief effort and help determine how American society as a whole judges itself.
Should New Orleans even be there?
For those of us who live in temperate climates it is easy to forget the huge efforts needed to protect cities and communities in places where nature is not benign.
The levees or dykes which protected, or were designed to protect, New Orleans were the equivalent to engineers of the Great Wall of China.
There have been innumerable warnings.
Last year the National Geographic magazine wrote about a disaster simulation which predicted that 50,000 people might die in the city in a Category Five Hurricane, which Katrina was for a time.
"The chances of such a storm hitting New Orleans in any given year are slight, but the danger is growing," the article said. "Climatologists predict that powerful storms may occur more frequently this century, while rising sea level from global warming is putting low-lying coasts at greater risk."
"It's not if it will happen," University of New Orleans geologist Shea Penland was quoted as saying. "It's when."
And National Public Radio had a very prescient documentary which interviewed Walter Maestri, head of public emergencies in Jefferson County or Parish.
After the simulation exercise, he wrote something in large letters across a map of the affected area: "KYAGB-kiss your ass good bye," it said.
But the answer to the question of New Orleans' future is that just as San Francisco was rebuilt, so will New Orleans be.
The levees will be made bigger and stronger. American engineers will not give in. They tamed the Mississippi on its run to the sea. They aim to tame it there as well.