It was, everyone agreed, a lovely summer. Here in the East, a warm sun filled the long days, and if the lawns seemed a little tawny and dry, it was a small price to pay; the rest of life was painted in brilliant hues and crowded with the delighted yelps from the shore, lakesides and mountains.

Labor Day isn’t quite here yet, but all that already seems so far, far away. The catastrophe in the Mississippi Delta and the skyward procession of gasoline prices have brought a sobering reminder that now it is back to business – and that life for the next several months, and perhaps years, will be a nasty business.

This was not the first time that a stunning summer ended with a stunning event with a sobering effect. It is difficult to quantify such matters, but everyone agrees that the summer of 1914 was a stunner as well; it was followed by the outbreak of World War I, and it is no coincidence that the most incisive recent interpretation of those events, by the historian David Fromkin, bears an ironic title: “Europe’s Last Summer.”

We all also remember the summer of 2001, when our leisurely idyll was disrupted only by worries about shark attacks in the Atlantic. That sabbatical of the senses ended on a Tuesday morning in the second week of September.

I am not arguing that beautiful summers bring grim Septembers, for the pinions of the human world do not rest completely on the caprices of nature.

If they did, the 19th-century bromide that rain follows the plow – an article of faith on the plains, especially in central and western Nebraska – might never have been dismissed as faulty climatology, and the banks of the Platte River might be chock-a-block with settlement today. But the events of the past week have heightened our awareness that, despite all of humankind’s efforts to harness and to conquer nature, the physical world has not lost all its power for revenge and damage.

That is but one of the lessons offered by the philosopher who bore the name of Katrina – a deadly force of nature that forced us to recall our dependence on nature and, now that you mention it, one of the byproducts of nature, oil. From this relatively arid perspective, here are some the other teachings of Katrina:

National security has many forms. Precisely four years ago this month, we altered our definition of national security, replacing our fears of attack from established nation-states with threats from determined and disciplined bands of the aggrieved. We swiftly became convinced that the greatest threat to our national security came from terrorists abroad or, ominously, at home. That threat has not diminished. But it should not have gone unnoticed that Michael Chertoff, who directs the homeland security operation, specifically referred to the aftermath of Hurricane Katrina as a national security issue. No one disputed the claim.

A complicated society has many choke points. The dramatic price run-ups of the past month, culminating with the ever-steeper price climbs of recent days, remind us of the power of a single natural resource to affect our entire way of life. But now we know that there are other choke points in our economy, and one of them is New Orleans, which in folklore is celebrated for its devil-may-care insouciance but in the American economy is essential for its role in the energy, grain and shipping industries. Choke off New Orleans and you choke the American economy.

Panic begets panics. Oil panics in 1973 and 1979 made the economy stagger – the hoarding was so bad that, in Japan, consumers hoarded not only gasoline but, inexplicably, toilet paper – and there are legitimate worries now that $3-a-gallon gasoline will choke (that word again) off the recovery. Perhaps it will. But now it is time for Americans themselves to play a role in their economic futures – by having the discipline not to tumble into panic and thus to drag down the economy with their own spirits. A generation ago, the nation flirted with economic ruin in part because of inflationary expectations. This generation faces a similar challenge – to realize that despite the horrible prospects being sown in conversation and on the Internet, the nation is better able to react to an oil shock now than it was in the last century.

The energy crisis is still with us. Despite our greater insulation to spikes in energy prices, Americans still are frighteningly vulnerable on the energy front. We still are dangerously dependent upon foreign oil and, worse yet, on the political and economic stability of the most instable region of the world.

Jimmy Carter was ridiculed three decades ago when he referred to the drive for energy independence as the moral equivalent of war, a phrase that came from a speech William James delivered at Stanford in 1906 but that, in a typical example of Carter’s bad luck, had a pussy-cat acronym (MEOW). Now that we are literally at war in an oil-rich part of the world, and fighting a shadow war against terrorists whose ideology of violence was sown in part by the disruptions that oil money brought to the region, we regret that we didn’t fight a moral war – perhaps then we might have been able to avoid fighting a real one.

There is a moral resilience to rebuilding. Americans, who after all named one of their cities Phoenix, rebuilt Chicago after the great fire, Galveston after the great hurricane and San Francisco after the great earthquake. Each city emerged stronger and better fortified for the future. So it will be with New Orleans.

Let the new New Orleans stand as a symbol of our resolve – and of our resolve to learn from this season of sobriety.