The terrorist attacks on the United States two decades ago brought profound change in America and the world.
The death and devastation stirred grief, rage and war. It also sparked solidarity, not only in the United States but among its allies – even some rivals.
World affairs reordered abruptly on that morning of blue skies, black ash, fire and death.
In Iran, chants of “death to America” quickly gave way to candlelight vigils to mourn the American dead.
Vladimir Putin weighed in with substantive help as the US prepared to go to war in Russia’s region of influence.
From the first terrible moments, America’s longstanding allies were joined by long-time enemies in that singularly galvanising instant. No nation with global standing was cheering the stateless terrorists.
For the US, the zero hour of September 11 2001 meant a chance to reshape its place in the post-Cold War world from a high perch of influence and goodwill.
This was only a decade after the Soviet Union’s collapse left America with both the moral authority and the military and financial muscle to be unquestionably the lone superpower.
Those advantages were soon squandered. Instead of a new order, 9/11 fuelled 20 years of war abroad.
In the US, it gave rise to the angry, aggrieved, self-proclaimed patriot, and heightened surveillance and suspicion in the name of common defence.
It opened an era of deference to the armed forces as politicians pulled back on oversight as presidents gave primacy to the military over law enforcement in counterterrorism. It sparked anti-immigrant sentiment, primarily directed at Muslim countries, that lingers today.
What most nations agreed was a war of necessity in Afghanistan was followed two years later by a war of choice as the US invaded Iraq on false claims that Saddam Hussein was hiding weapons of mass destruction.
Thus opened the deep, deadly mineshaft of “forever wars”.
Convulsions ran through the Middle East and US foreign policy gave way to a head-snapping change from George Bush to Barack Obama to Donald Trump. Trust in America’s leadership and reliability waned.
Other parts of the world were not immune. Far-right populist movements coursed through Europe and China steadily ascended in the global pecking order.
Now, President Joe Biden is trying to restore trust, but there is no easy path. He is ending war, but what comes next?
In Afghanistan in August, the Taliban seized control with menacing swiftness as the Afghan government and security forces that the US and its allies had spent two decades trying to build collapsed.
No steady hand was evident from the US in the disorganised evacuation of Afghans desperately trying to flee the country.
In the United States, the 2001 attacks had set loose a bloodlust cry for revenge.
A swathe of American society embraced the binary outlook articulated by president Bush — “Either you are with us, or you are with the terrorists” — and has never let go of it.
Factionalism hardened, in school board fights, on Facebook posts, and in national politics, so that opposing views were treated as propaganda from mortal enemies. The concept of enemy also evolved, to include immigrants as well as terrorists.
The patriot under threat became a personal and political identity. Mr Trump would harness it to help him win the presidency.
For the US, the presidencies since Mr Bush’s wars have been marked by an effort to pull back the military from the conflicts of the Middle East and Central Asia.
The perception of a US retreat has allowed Russia and China to gain influence in the regions and left US allies struggling to understand Washington’s place in the world.
The notion that 9/11 would create an enduring unity of interest to combat terrorism collided with rising nationalism and a US president, Mr Trump, who spoke disdainfully of the Nato allies that in 2001 had rallied to America’s cause.
To be sure, the succession of US presidents since 9/11 scored important achievements in shoring up security, and so far US territory has remained safe from more international terrorism anywhere on the scale of September 11.
Globally, US-led forces weakened al Qaida, which has failed to launch a major attack on the West since 2005. The Iraq invasion rid the world of a murderous dictator in Saddam Hussein.
Yet deadly chaos soon followed his overthrow. The Bush administration, in its nation-building haste, had failed to plan for keeping order, leaving Islamist extremists and rival militias to fight for dominance.
Today, the legacies of 9/11 ripple both in obvious and unusual ways.
Most directly, millions of people in the US and Europe go about their public business under the constant gaze of security cameras while other surveillance tools scoop up private communications.
The government layered post-9/11 bureaucracies on to law enforcement to support the expansive security apparatus.
Militarisation is more evident now, from large cities to small towns that now own military vehicles and weapons that seem well out of proportion to any terrorist threat. Government offices have become fortifications and airports a security maze.
But as profound an event as 9/11 was, its effect on how the world has been ordered was temporary and largely undone by domestic political forces, a global economic downturn and now a lethal pandemic.
