By Paul Adams
BBC News, Washington
The death of America's most wanted man, Osama Bin Laden, was greeted by jubilant crowds across the US, releasing 10 years of anger - but other voices warn that the work of protecting the US from Islamist militants is not yet over.
While American special forces in Afghanistan were putting the final top secret touches to their preparations for a raid that would carry them deep into Pakistan, I was, of course, blissfully unaware.
The 9/11 attacks resulted in a 10 year manhunt for Osama Bin Laden
But I was in the midst of trying to do something not unrelated - explaining the horror of 9/11 to my two young sons.
We were at Ground Zero, in New York, where for many it all began on that pristine September morning, 10 years ago.
The site today is a frenzy of construction - earth movers and cranes, concrete mixers and scaffolding, and hundreds of men in hard hats.
Next to the site of the old Twin Towers, the building that will soon come to dominate the lower Manhattan skyline - the building known colloquially as Freedom Tower is steadily rising above its neighbours.
But it is not yet the new that people come to ponder, it is still the destruction of the old - the magnificent, graceful Twin Towers, brought down in a cataclysmic roaring cascade, the memory of which still has the power to terrify and appall.
At the nearby visitors centre, set up as a tribute to the towers and the 2973 people who died here, at the Pentagon and in the Pennsylvania countryside, my sons watched the videos and examined the exhibits with uncharacteristic solemnity.
The pictures of the missing, the voices of relatives and the fragments recovered from the wreckage - a credit card, a photograph, a set of mini-dictionaries; it all helped to explain something they have heard their parents talking about but not, until now, really had a chance to appreciate for themselves.
The sound of a nation wildly celebrating... but also of people, together, releasing a decade of pent up anger and bitterness
At the end, they sat down to write something on the slips of paper offered for comments.
Both chose a single word.
William, his imaginative 12-year-old mind racing, wondered, "why?"
Felix, three years younger and more perplexed, asked simply, "how?"
All around us, visitors, from New York and beyond, moved silently through the museum, variations of these questions running through their minds, many faces still wearing the same expressions of mute horror that filled our screens a decade ago.
Back home, on Sunday, around the time that the rotor blades of two American Black Hawk helicopters began to turn at a base in Afghanistan, the boys wanted to know more.
With some trepidation, on their behalf but also mine, I let them watch the moments when the towers came down - images deliberately avoided at the museum and rarely seen any more on TV.
They are terrifying enough, but the howls of disbelief and fear emanating from the mouths of unseen onlookers are equally disturbing.
Somehow, it was a sound I had allowed myself to forget.
Moment of catharsis
A few hours later, I was listening to different voices on my television - whooping, cheering, bellowing voices from the streets outside the White House, from Ground Zero and from a baseball game in Philadelphia.
President Obama watched nervously as troops stormed Bin Laden's hideout
The sound of a nation wildly celebrating, for sure, but also of people, together, releasing a decade of pent up anger and bitterness against a man who wounded them so grievously and then seemed to vanish into thin air.
The chants of "U-S-A! U-S-A!" and the pumping fists felt, at times, like mindless jingoism, but this was a moment of national catharsis and it was an undeniably powerful spectacle.
The president, when he appeared on television, to confirm what millions of Americans already knew, was typically dignified.
Images released by the White House showed Barack Obama and his national security team watching the operation unfold in the situation room, in the basement of the White House.
The sense of tension, as Barack Obama stares intently at the screen and Hillary Clinton holds her hand to her mouth, is palpable.
Amid the sense of euphoria... there were plenty of voices urging caution
But it seems that when it was over and the success of the mission was confirmed, the president was not above a little historically resonant flourish.
"We got him," he apparently said, using the phrase immortalised by Paul Bremer, George Bush's man in Baghdad in late 2003, when Saddam Hussein was pulled from his spider hole near Tikrit.
Amid the sense of euphoria generated by the news from Abbottabad - a name the satirist Jon Stewart said was just what a New Yorker would call a town in Pakistan: "yeah, Abboddabad" - there were plenty of voices urging caution.
The president himself said the death of one man did not mean the end of the war on al-Qaeda.
And a soldier, interviewed on the radio, summed it up with the kind of simple eloquence Americans do so well.
"Yeah, we got him," he said. "But we still got to go back and get someone else."
That sense of a mission apparently without end is still a powerful one, even as the president prepares, in the next couple of months, to lay out the path towards an American withdrawal from Afghanistan.
The death of Bin Laden will help Mr Obama to make the case that a significant part of America's work is done.
The man who masterminded the 9/11 attacks has gone to an anonymous, watery grave with a bullet in his brain and Afghanistan is no longer much of a haven for al-Qaeda.
Rising to a challenge
There is plenty of other business, to be sure, but the reason America went there in the first place is all but gone.
The seemingly impossible task of capturing Bin Laden has now achieved
The havens are elsewhere and the war on terror has many other fronts, abroad and at home.
As they debated the significance of this week's momentous news, members of the Senate Foreign Relations Committee wondered if it really made sense to be spending billions of dollars in Afghanistan when Yemen and, perhaps Pakistan, seemed to pose more of a threat.
But for all the worries, it is undoubtedly the case that America is feeling good about itself.
"Let us remind ourselves," Hillary Clinton said on Monday, "this is America. We rise to the challenge, we persevere and we get the job done."
And perhaps because last Friday should have marked the final launch of the Space Shuttle - another great symbol of American perseverance in the face of adversity - I found myself hearing echoes of President Kennedy, urging America to put a man on the moon before the end of the 1960s.
"We chose to do these things," he said in September 1962, "not because they are easy, but because they are hard."
Back then, the thought of a man walking on the moon by the end of the decade seemed wildly ambitious.
As the 1960s wore on, it because more and more tangible.
But over the years since 9/11, the opposite thing happened - an eminently do-able task, finding a villain in a cave, turned into something apparently impossible.
Until now, when suddenly perseverance has paid off.
America has, indeed, risen to the challenge.
That it may, along the way, have involved water-boarding, rendition and black sites is a question that some are already asking.
But I suspect that a majority of Americans would argue, now, that finally getting America's most wanted man was well worth the price of a little moral turpitude.
And so, in a way, the 21st Century's first decade is over, book-ended by two defining events in the struggle we came to call the war on terror.
History will, I suspect, record that the war went on rather longer.
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