Around the globe yesterday, the atrocity that changed the world was commemorated with due dignity and solemnity. And nowhere outside the United States was it reflected on with deeper emotion than in Ireland.
Almost 3,000 people were killed in the terrorist attacks of September 11, 2001, when three hijacked aircraft were deliberately flown into the World Trade Centre in New York and the Pentagon in Washington. The great majority of victims died in the Twin Towers of the World Trade Centre. They included 341 firefighters and two paramedics who gallantly entered in an attempt to rescue people from the devastated buildings.
They were not the only heroes. The passengers on a fourth aircraft fought the hijackers, causing the plane to crash in Pennsylvania.
But the firefighters, like the New York police, were our kin in emotional terms, sometimes in literal terms. This resembled a family tragedy as well as a world tragedy.
Here, as on every continent, there was an outpouring of grief and sympathy for the US. One French newspaper summed it up in the headline: "We are all Americans."
Then the world waited for the United States' response. When it came, it was almost as astonishing as the atrocity itself.
The 9/11 attack was planned and executed by a group of highly educated young men, mostly Saudi Arabians, from privileged backgrounds. They were motivated by a fanatical hatred of everything Western, and blamed the West for all the ills of the Muslim peoples. Their leader, Osama bin Laden, believed in a holy war against the West, ending in its destruction and the establishment of a world-wide Islamic caliphate.
He was sheltered by the Taliban in Afghanistan. When Washington demanded his surrender, the Taliban refused. The United States invaded Afghanistan, and President George W Bush went on to commit a series of shocking blunders which, in total, turned the wave of pro-American feeling, especially in Europe, into a flood of opposition and condemnation.
He declared himself engaged in a "crusade": the term, of all terms, certain to anger Muslims. He claimed that he invaded Iraq, in addition to Afghanistan, to force the dictator Saddam Hussein to surrender weapons of mass destruction. No weapons were ever found, but the death toll in Iraq ran into hundreds of thousands.
In the United States and elsewhere, governments rushed to enforce enhanced security measures. For most Western citizens, they meant little more than inconvenience at airports. But they were accompanied by sharply increased military and security spending -- and in the context of economies suffering from uncertainty and from the costs of the wars in Afghanistan and Iraq. The overall cost to the American economy has been calculated at three trillion dollars, a figure almost beyond imagination.
Among the political consequences have been the success of al-Qa'ida in establishing itself in Iraq -- where previously it had not existed -- and the rise of Iran, led by a regime both eccentric and militant, to the status of a leading regional power with an increased capacity to promote instability.
Yet in the midst of all these misfortunes, the unquenchable force of the human spirit has lately manifested itself in an unmistakable way. And the inspiration has not come from the West, but from within the Muslim world.
The "Arab Spring" revolutions, beginning in North Africa and moving to the Middle East, had mixed origins and have developed erratically. There is no guarantee that any of them will succeed in the longer term, and there is great doubt about the nature of the regimes that emerge from them. One certainty is that Islamic political influence will increase in most countries of the region and Islamic parties, some radical, will form at least part of several countries' governments.
But the relationship between the West and the Muslim world cannot be defined, much less determined, by inexperienced revolutionaries desperately trying to bring stability to countries afflicted by the legacy of dictatorship.
Whatever the challenges to the primacy of the United States, it remains the world's leading power and, supremely, the leading democracy.
Respect and affection for the US dwindled under the Bush presidency, appeared to revive with the election of President Barack Obama, and have lately dwindled again.
Only one man can rekindle that flame. In what remains of his first term -- which may turn out to be his only term -- President Obama must find it in himself to provide the leadership the world so sadly lacks. There could be no better way to honour those who died on September 11, 2001.