ASHINGTON, Sept. 11 — Today's devastating and astonishingly well-coordinated attacks on the World Trade Center towers in New York and on the Pentagon outside of Washington plunged the nation into a warlike struggle against an enemy that will be hard to identify with certainty and hard to punish with precision.

The whole nation — to a degree the whole world — shook as hijacked airliners plunged into buildings that symbolize the financial and military might of the United States. The sense of security and self-confidence that Americans take as their birthright suffered a grievous blow, from which recovery will be slow. The aftershocks will be nearly as bad, as hundreds and possibly thousands of people discover that friends or relatives died awful, fiery deaths.

Scenes of chaos and destruction evocative of the nightmare world of Hieronymus Bosch, with smoke and debris blotting out the sun, were carried by television into homes and workplaces across the nation. Echoing Franklin D. Roosevelt's description of the attack on Pearl Harbor as an event "which will live in infamy," Gov. George E. Pataki of New York, a Republican, spoke of "an incredible outrage" and Senator Charles E. Schumer of New York, a Democrat, spoke of "a dastardly attack."

But mere words were inadequate vessels to contain the sense of shock and horror that people felt.

As Washington struggled to regain a sense of equilibrium, with warplanes and heavily armed helicopters crossing overhead, past and present national security officials earnestly debated the possibility of a Congressional declaration of war — but against precisely whom, and in what exact circumstances? Warships were maneuvering to protect New York and Washington. The North American Air Defense Command, which had seemed to many a relic of the cold war, adopted a posture of heightened alert and suddenly seemed relevant.

Disappointing some of his political advisers and allies, who felt he should have returned to Washington at once from a trip to Florida to symbolize that the government was functioning, President Bush headed instead to Offutt Air Force Base near Omaha, where a more secure command post was available. At the height of the Cuban missile crisis, when the sense of danger was just as palpable, John F. Kennedy stayed in Washington.

The president flew back to Washington tonight, escorted by F-15 and F-16 fighters, as leaders of both parties closed ranks behind him with pledges of support and a stirring rendition of "God Bless America." In a brief, earnest televised speech, he said the day had taken "thousands of lives" and generated "a quiet, unyielding anger" in the nation. He promised that those who harbored terrorists would be treated as harshly as the terrorists themselves.

For Mr. Bush the attacks constituted a threat and an opportunity. A minority president, just a few months into his term, derided by many as intellectually inadequate for his job, he is likely to be judged, at least in the months ahead, on whether he can take command and act decisively.

Will he prove to be a Jimmy Carter, whose presidency was poisoned by his inability to resolve the Iranian hostage crisis? Or will he enhance his reputation, as Ronald Reagan did after the explosion of the space shuttle Challenger and as Bill Clinton did after the Oklahoma City bombing?

Samuel Popkin, a political scientist at the University of California at San Diego, who worked in the presidential campaign of Al Gore, said: "Many Americans have come to consider politics irrelevant in recent years. Now politicians matter again, and the president, in his role as commander in chief, becomes our focal point."

In a statement made at an air base in Louisiana, Mr. Bush said that "the resolve of our great nation is being tested" and pledged that the test would be met. He is being tested as well, far more severely than ever before.

Senator John McCain of Arizona, Mr. Bush's sometime Republican rival, said that he was confident the culprits would be caught and severely punished and that the president would "ensure something like this will never happen again." Together, the two men's comments set the bar very high — perhaps too high.

No doubt the public will rise to the challenge as best it can; many of those in the buildings hit today reacted with almost supernatural calm. But Mr. Bush alone must decide how to retaliate and against whom, and he will be operating in a murky area.

"This went far beyond anything we had expected," a senior intelligence official conceded. "We're better than we used to be at monitoring terrorist activities, but today makes it obvious we are not nearly good enough."

It follows, then, that preventing another attack will be very difficult. And while it is evidently easier to identify malefactors after an attack than it is to predict their activities, many experts cautioned against assuming, for example, that today's attacks were the handiwork of Osama bin Laden.

Even if Washington concludes that his organization or some other foreign terrorist group was responsible, devising an appropriate response will present a number of complications. For one thing, bombs and rockets tend to kill the innocent as well as the guilty. As a Democratic senator said, "You're likely to bomb a city or a village with 100 terrorists and end up with 400 or 500 when the warplanes have flown away."

Some strategists suggested that Mr. Bush might mount a ground attack on bin Laden's headquarters, which are believed to be somewhere in Afghanistan, much as Woodrow Wilson sent American troops into Mexico before World War I to capture Pancho Villa dead or alive. But they did not get him. And Afghanistan has proved resistant, throughout its history, to foreign forces trying to operate there.

If the United States develops solid evidence that any country aided the perpetrators of today's attacks, said Richard C. Holbrooke, ambassador to the United Nations under the Clinton administration, a declaration of war against that country might be appropriate and retaliation against it should certainly be undertaken in short order.

Another consideration is the attitude of the Middle Eastern oil states to any retaliatory attack by the United States, which remains uncertain.

Many Americans, particularly those old enough to remember Dec. 7, 1941, compared today's events to the Japanese attack on Pearl Harbor. But then, after the first few minutes of confusion it was evident that the planes were Japanese — they had military markings on their wings, instead of airline names on their fuselages, as was the case today. Militarily daring as the Doolittle raid on Tokyo months later may have been, there was no doubt that it hit the right target.

This was Pearl Harbor redux without the face of an enemy. In today's more anomalous situation, Mr. Bush is likely to avail himself more fully than he has to date of the knowledge, experience and prestige of his secretary of state, retired Gen. Colin L. Powell. The president "needs Colin like he's never needed him before," an administration official said.

It appears possible that the attacks will undercut Mr. Bush's campaign for a missile defense shield by suggesting that such a shield would concentrate American resources on seeking protection against the wrong kind of threat. People who wish the United States ill "don't need missiles to do this country harm," Professor Popkin argued, "and we saw that clearly this morning."

On the other hand, the attacks might persuade the American public that everything possible should be done, no matter what the cost, to protect the nation in any way possible.

It will be up to Mr. Bush to make that case. If he makes it well, it will be harder for the Democrats to mount a partisan assault on his plan.

It will be up to Mr. Bush as well, perhaps even more important, to keep the country from entering a kind of psychic bomb shelter, reluctant to make decisions, take chances, even to fly from one city to another after the four hijackings involved in today's campaign of terror. One of the most important contributions to Londoners' morale during the Battle of Britain was made by King George VI, who calmly toured the city despite the blitz.

Inevitably, the attacks will make daily life in the United States more complicated. Security will be tightened at private buildings and federal offices. Airport checks will be stricter and more frequent, requiring passengers to arrive earlier. In general, it will be harder to get about.

At past moments of national tension, like the Communist challenge following World War II, civil liberties have come under pressure. Senator Joseph R. Biden Jr., Democrat of Delaware, warned today that "if we alter our basic freedoms, our civil liberties, change the way we function as a democratic society, then we will have lost the war before it has begun in earnest."

And what of the nation's economy, the main preoccupation of Washington, the president and much of the nation until this morning? Although the stock market remained closed today, Karen P. Hughes, Mr. Bush's counselor, was at pains to state at a briefing this afternoon that the rest of the economic system functioned normally. But it was by no definition a normal day, and sooner or later its hellish events are bound to affect the economy in some way — probably not for the better.