It makes no difference who you are or what your job was, we all recall what we were doing when we first learned of the 9/11 attacks.
I was in Brighton, preparing to address the TUC Congress. I liked to be left alone in the run-up to a major speech to make final touches to the words.
So I knew immediately it had to be important when I was told I must look at the live pictures on TV.
What I saw – smoke billowing from one of the towers of the World Trade Centre – was hard to take in.
It was still not absolutely clear whether what we were all watching was the result of an inexplicable accident.
Minutes later, as another aircraft was flown deliberately into the second tower, all doubt disappeared.
I remember sharing the profound sense of shock and horror. But I realized, too, immediately that our world had changed.
This was a declaration of war not just on the United States but all countries and peoples which treasured freedom, democracy and our shared way of life.
The decision was taken to return immediately to London. Back in Downing Street, I began an immediate round of meetings with ministers, officials and security chiefs.
As more details emerged including news of the aircraft flown into the Pentagon, it was clear these were meticulously planned and choreographed attacks.
There was an obvious fear that the UK itself might become a target. I had phone calls with my counterparts across the world.
I spoke in turn to the leaders of Germany, France and Russia – and to President Bush the next day. It is right we remember how we felt that day because that feeling shaped the events that followed.
There was an extraordinary sense of solidarity, a common determination to stand ‘shoulder to shoulder’ with the United States.
The World Trade Center was, after all, as an international a workplace as possible with employees, as we were to learn over the coming days, from almost every country, faith and background including many of our own citizens.
And it was the indiscriminate nature of these attacks, their savagery and scale which convinced me right from the start of the lasting impact of 9/11.
The death toll may have made it the worst terrorist outrage in history.
But it was clear that if those who masterminded these attacks could have slaughtered not 3,000 but 300,000 they would not have hesitated.
We did not choose this fight nor provoke it. But it was one we could not shirk.
This remains the case today. As I have said, if I had known a decade later that we would still be fighting in Afghanistan and the tragic continued loss of life, I would have been profoundly alarmed.
But what I now know does not make me any less committed to the fight we began that day or doubt what is at stake.
We have had successes against those who planned these attacks but the extremist ideology which drove their actions still exists.
The threat to our way of life, the decent values we hold and to our peace and prosperity remains.
While the number of extremists is small, we underestimated the numbers who share the narrative of radical Islamism and who believe they are in fundamental conflict with us who do not share it.
The majority, as the Arab Spring show, want what we want. But the minority are well organized and very determined and they are not confined to the ranks of suicide bombers.
It will take a generation of effort at many levels including, importantly, to support open-minded and tolerant people of all religions to change hearts and minds and make the fanatics irrelevant.
Some people naturally want to say that the answer to this lies in the realm of politics; and of course politics has a crucial role to play. But it is clear that since the dimensions of this struggle are inevitably affected by religion itself, the argument in favor of the open approach has to go wider and deeper than simply asking people to behave nicely to one another.
It has to address full-on, the spiritual, theological and scriptural basis for mutual respect towards those who follow a different religious or spiritual path.
Practical and genuine interfaith work is a powerful counter-argument to the extremists’ recruiting narrative that their religion is disrespected and under threat.
Through the work of my Faith Foundation, I’ve seen how positively young people respond when given the opportunity to create alliances across different cultures, nationalities and religions.
Ten years ago, the certainties of our world were turned upside down in a few terrible minutes.
We must find the determination to remake the world in a way in which all feel they belong.
Tony Blair is the founder and patron of the Tony Blair Faith Foundation.
More On Faith and 9/11:
Desmond Tutu: Our post-9/11 failures
Tony Blair: Remaking the world after 9/11
Sam Harris: 9/11 demands intellectual honesty
Thomas Monson: Rebuilding our souls
T.D. Jakes: Spirituality after the attack
Feisal Abdul Rauf: Radical Islam on its way out
Donald Wuerl: Peace begins internally
Katharine Jefferts Schori: Live the memorial
Mark Driscoll: Death and the hope of resurrection
Karen Armstrong: Unite through compassion
Deepak Chopra: Divided hearts, divided world
Yasir Qadhi: Americans still don’t know Islam