By JOHN PODHORETZ

FOR the second time in four years, the United States has been changed utterly by a previously unthinkable event. And just as was the case after 9/11, how this nation responds to the deluge that is sweeping New Orleans away will help define the nature of its character for decades.

Just as Rudy Giuliani said that the death toll from 9/11 would be more than any of us can bear, the same is already true of Katrina. Who can begin to take in the notion that in the United States in the 21st century, a storm could kill in staggering numbers?

At the beginning of the 20th century, something like 8,000 people perished when Galveston, Texas — unprotected from storm swells at the time — was hit by a hurricane. But when Hurricane Andrew leveled the entire town of Homestead, Fla., 13 years ago and became the most financially deadly storm in American history, it took only 15 lives.

Now we're talking about several hundred times that number in the literal swamping of one of the world's great cities.

There can be no doubt that the immediate response will be one of breathtaking generosity — financial, spiritual and personal. That's what we saw in the wake of 9/11, it's what happened after the tsunami in December, and it's what we will begin to see as the next few days pass.

But what we don't yet know is this: Are we going to try to look forward, to figure out how to save New Orleans and prevent another calamity of this sort there and elsewhere? Or are we going to begin finger-pointing, searching for villains among the debris?

Some of that villain-hunting has already begun, in the typically vulgar, unwisely speedy efforts made by overly assured ideologues certain that they can connect a cataclysm to a pet issue — whether it be the American failure to pass the Kyoto global warming treaty or making the claim that spending on the war in Iraq squeezed out the possibility of shoring up the New Orleans levees.

Here we see the stirrings of a spiritual divisiveness taking hold — in the form of a know-nothing populism that sweeps everything in its wake and brings everything into the courtroom.

What happened here was a natural disaster. But there will be the temptation to turn it into a human conspiracy of greed and selfishness on the part of oil companies, concrete companies, politicians, insurers, re-insurers, goonish cops and the like.

If the recriminations become the story of the next months, everybody will simply go to the usual battle stations. The tort reformers will take on the trial lawyers. The global-warming crowd will face off against American business. The politicians will scream at each other, scoff at each other, and try to find some cheap advantage that will turn the tide against one party or the other.

The good that will be done —person by person, donation by donation, community by community — will be in danger of getting swamped by the bitterness and divisiveness that characterizes contemporary elite politics. Rather than finding common ground, there will be ugly partisanship and a cold standoff.

The horror of a flood is literally, very nearly the oldest story in the Book. There have always been times that the water will rise higher than the walls men can build to contain it. The New Orleans system survived the battering of nature for more than 200 years — but it met its match and was overwhelmed by it.

The best we can do is comfort the afflicted, mourn the lost, and try to rebuild. The worst we can do is turn on each other.

So what shall it be? E-mail:

podhoretz@nypost.com