When American Airlines Flight 11 from Boston to Los Angeles took its deadly detour on September 11th and smashed into the north tower of the World Trade Centre in Manhattan, American society was wrenched out of joint. An era ended on that beautiful late summer morning and America crossed a line into a new, and uncertain phase of its history.
Those of us who witnessed the beginning of what President George Bush called the "first war of the 21st century" will forever recall images of utmost horror.
As Wall Street correspondent I was working in my 41st-storey home office, writing an article about failing New York restaurants, when the first fireball exploded at 8.48 a.m. from the north tower four blocks away, leaving a jagged hole several stories high and showering debris onto Church Street far below.
I thought at first it was a massive bomb, then realised it had been a plane. As the flames took hold a man appeared at a window hanging out over a drop of 80 stories, waving a white cloth.
I was watching him when our house guest, Dublin psychology student Aoife Keane (20), on her very first day ever in New York, said in disbelief, "There's another plane." United Flight 175, also en route from Boston to Los Angeles, tilted as it skimmed in over the Hudson River and smashed into the south tower, propelling a massive orange ball of flame and smoke through the other side.
During the next hour, we could only watch helplessly as dozens of heart-wrenching individual tragedies were enacted within our helpless gaze. More people appeared at upper storey windows crying soundlessly for assistance. We saw five fall to their death, spreadeagled like rag dolls. The south tower collapsed first, smashing down on fire engines and ambulances and crushing the tiny figures of New York firemen attempting in vain to run for their lives on West Side highway.
Over 300 firefighters, scores of them Irish Americans who marched on St Patrick's Day in Emerald Society bands, were to die before the morning was over.
The man with the white cloth clung to life for well over an hour until the massive structure suddenly collapsed beneath him, swallowing his tiny figure up in a black inferno of debris, dust and smoke which spread out across downtown Manhattan preceded by a warm blast of air.
Throughout the afternoon, residential buildings nearby were evacuated. We were the last of the 1,000 people in the 44-storey Tribeca Pointe Building to depart as night fell, ordered out by the police. The electricity, water and gas had been cut off at 4.00 p.m. and the whole of south Manhattan below 14th street was closed to normal traffic.
My wife Zhanna and I and our house guest, with a couple of days' change of clothing in shoulder bags, trudged north through streets clogged with emergency vehicles and exhausted firemen, looking for a hotel room, thinking we would be back inside 24 hours.
Beyond that the streets of the city that never sleeps were eerily quiet, though in Greenwich village we saw a few people eating at small open-air wine bars, including actress Helen Hunt, in surreal semi-normality. Chris Coyne, a worker from St Vincent's Hospital, gave us a lift uptown as he came off a long shift.
The hospital had been ready for many more casualties than they had received, he said. It was a depressing sign. The awful truth was that those caught inside the collapsing towers had mostly been killed, though we learned later that tens of thousands had managed to escape.
For some time it was difficult for us to connect what we had seen with reality. But the full, emotional impact of the most deadly act of war against the United States on American soil was brought home to us on Wednesday and Thursday when relatives of the missing appeared outside downtown hospitals.
From the pictures they carried we could put faces to the workers who had been trapped inside the towering infernos before they collapsed and were still unaccounted for, people like Angela Susan Perez, whose niece Beth Fitzsimmons passed out flyers with her picture and an appeal to call her home telephone number.
They stood in sad silent rows, some weeping, as they showed snap shots to TV cameras, without real hope, but refusing to give up.
We heard stories of last minute telephone calls from stricken company executives and office workers dialling frantically from Trade Centre offices to their homes before the lines went dead and the screens went blank.
We heard of how randomly death came and was thwarted; how Nat Alcomo of Morgan Stanley Dean Witter was told by his fiancΘe to flee and made his way down from the 60th floor, ignoring an official advising people to stay put, just moments before the building was struck; how Richard Jacobs of Fuji Bank got out while several colleagues returned to their 79th floor office after hearing the situation was under control; how Monica O'Leary was laid off just on Monday by her company on the 105th floor.
One of the most extraordinary stories of all was of Pat and Lillian Vallone whose daughter Francesca worked in the Trade Centre and son Joe in the Pentagon.
As Joe talked by telephone from his Pentagon office to his frantic mother who was watching the Trade Center burn from her Brooklyn home, American Airlines Flight 77 from Dulles to Los Angeles crashed into the building behind him, throwing him from his chair as it obliterated part of the US military command centre.
Both survived.
In the aftermath of what the television programmes label "Attack on America" the people of New York and all of the United States learned that while the purpose of terrorism is to terrorise, the secondary fall-out is loss of freedom.
For several days the freedom to travel anywhere in the United States was severely curtailed to prevent further attacks. All airports were closed and the thousands of routine flights criss-crossing the United States were ordered to terminate at the nearest airport.
Air travellers were left stranded whererever they happened to be, husbands, wives, sons, daughters, business executives, visitors from abroad, no one could continue their journeys.
Major bridges from the Golden Gate in San Francisco to the George Washington Bridge in Manhattan, were blocked off. The New York Stock Exchange was shut down for the longest number of consecutive days since it was founded in 1792.
Major League baseball games were cancelled. The lights went out on Broadway as all theatres closed for two nights. Businesses, shopping malls, skyscrapers and government buildings were closed for a time until the fear of more attacks had ebbed.
New York got the jitters.
The streets were filled with wailing sirens. Armoured vehicles and soldiers in battle gear appeared in downtown streets and a tank was posted on FDR highway, where cars were stopped at checkpoints. False rumours were reported as fact on local television: a truck loaded with explosives had been stopped before George Washington Bridge; the Empire State Building had been evacuated because of a bomb.
The damage to New York as the world capital of finance, theatre, fine arts, dining, and so many other spheres of excellence has still to be calculated.
Four hundred and thirty national and 26 international businesses have been destroyed and the landmark towers which symbolised New York's rise from near bankruptcy in the 1970s have been torn from the heart of the Big Apple.
Insurers have estimated that the claims from the most costly man-made catastrophe ever inflicted on a modern city will amount to $40 billion. History teaches us that the markets, which were already reeling under the impact of an economic slowdown, will take a beating when they reopen on Monday.
But the cost cannot be measured in dollars alone. As the dust drifted away across the East River, it became clear that this free and open society risked losing some of its civil liberties as a result of its hellish experience.
As it was, the attack managed to stop democracy in its tracks, forcing the postponement of New York primary elections for several city posts, including mayor, which had been scheduled for September 11th.
Already we know from small but telling details - such as the banning of even plastic knives from airport terminals - that airline travel will never be the same again because of much tighter security.
Throughout it all New York Mayor Rudolph Giuliani was a constant presence on the streets and television screens, praising the firefighters and police, urging calm, appealing to the best side of New Yorkers.
"We are going to rebuild, we're going to come out of this stronger than before," he said.
Giuliani was, as a New York Times columnist put it, a leader who made the President of the United States look small and unscripted. George Bush, who did not come to the stricken city until yesterday, failed to give New Yorkers a sense this week of a man in charge.
"We need President Clinton to express our grief," cried a woman in a television interview.
President Bush was in Florida when the attacks came. Upon news of the strikes against the symbol of America's economic and military might he flew aboard Air Force One to a secure military base in Louisiana, and then on to Nebraska where he descended into a bunker and held a video conference with officials at the White House.
On returning to Washington on Wednesday he felt compelled to defend his inaction, saying it "as the commander-in- chief to be in a position to be able to make the decisions necessary for our government to handle the crisis". White House officials said anonymous callers had telephoned the Secret Service saying "Air Force One is next".
Vice-President Dick Cheney took charge in the White House but on Thursday he moved to Camp David as a precautionary measure as armed soldiers appeared to patrol, Belfast-style, the streets of Georgetown, the elite district of Washington where many prominent political figures live.
"This country will not relent until we have saved ourselves and others from the terrible tragedy that came upon America," said Mr Bush on Thursday, his lips quivering as he fought back tears. "Now that war has been declared on us, we will lead the world to victory - to victory." But he didn't say how.
Early opinion polls show that Americans favour a swift and forceful retaliation, even if it meant casualties, and in New York and elsewhere more people than usual queued to enlist in the armed services.
"When they can do this to New York, it's everybody's fight," said one applicant, reflecting a new spirit of patriotism which has driven people to buy up and display American flags all over the counrty.
Grief is giving way to cold anger. The entire political landscape has been altered. Anti-globalisation protesters have called off as inappropriate training sessions for the September 29th-30th meeting of the IMF and World Bank in Washington, which will anyway almost certainly be cancelled for security reasons.
In the coming days airliners will fill the skies over the United States again and trading in equities will resume on Wall Street, but in a more uncertain economic climate. Shares were falling before Tuesday, investment was drying up, unemployment was rising.
The day that shook Manhattan and the world will make everything so much worse. For America may have lost the elusive thing on which the markets depend and that is confidence, not just the confidence of consumers and investors, but confidence in itself.
Meanwhile, the several thousands of evacuated inhabitants of lower Manhattan will (I sincerely hope) be allowed to return to apartments to clean out refrigerators and remove abandoned pets that have died from hunger.
Our problems however are insignificant compared to the tragedy which ripped so many families apart, though life in downtown Manhattan will never be normal again for those of us to whom the towers were a constant, reassuring and awesome presence.