A new and deadly era began when the planes sliced into the twin towers on the morning of 11 September 2001. That evening, the historian Tony Judt wrote that he had seen the 21st century begin. The nearly 3,000 lives stolen by al-Qaida were only a small part of the toll. The horror began a chain of events that has claimed hundreds of thousands of lives, including huge numbers of civilians abroad and many US military personnel. It is still unfolding.

If the killing of the plot’s mastermind Osama bin Laden a few months before the 10th anniversary perhaps let some hope that an end to that new era might be in sight, there can be no such false confidence at the 20th. The establishment of a Taliban government in Kabul, two decades after the US ousted the militants for harbouring Bin Laden, has underscored two things: that far from reasserting its global supremacy, the US looks more vulnerable today; and that the echoes of 9/11 are still reverberating across the region – but will not stay there.

Al-Qaida itself survives and others claim its mantle. In the west, the threat from Islamist terrorism endures – from 7/7 and the Madrid train bombings, to the attacks at Manchester Arena, the Berlin Christmas market and Vienna – though the nature of the threat has shifted, from a heavily financed, complex and internationally organised plot to more localised, less sophisticated attacks. This week, 20 men went on trial over the 2015 massacre at the Bataclan concert hall and other sites in Paris. Ken McCallum, MI5’s chief, said on Friday that the agency had prevented six “late stage” terrorist plots during the pandemic, and that with the Taliban’s triumph, “more risk progressively may flow our way”.

The determination to pursue a military solution fed the political problems, as history should have warned. (A Rand Corporation study of 248 terrorist groups worldwide suggested that only 7% were ended by military force.)

In Afghanistan, the refusal to accept a Taliban surrender paved the way for America’s longest war and ultimate acceptance of defeat. Islamic State arose from the ashes of the invasion of Iraq. Extraordinary renditions, torture, the atrocities of Abu Ghraib and the unwillingness to acknowledge or atone for civilian deaths at the hands of US forces or their allies all stoked the fire. These abuses and crimes were not anomalies but intrinsic to the war on terror. Men swept up in the aftermath are still held at Guantánamo Bay.

Around the world, basic rights were erased at home too. The US saw a massive expansion of presidential power; the veneration of secrecy; the destruction of norms; the normalisation of Islamophobia; the promotion of a narrative linking immigration and terrorism, breeding broader intolerance; and the encouragement of the belief that ordinary citizens were in a state of war. It is not hard to draw the line to the rise of Donald Trump and white supremacy, or rightwing populism elsewhere. In the US, far-right terror groups were behind most attacks last year; in the UK, police have said that the fastest growing terror threat is from the far right. The biggest perils to the US now appear not external but internal. The future of a divided and distrustful country looks increasingly precarious, its status in the world weakened.

Whatever many in the country once believed, American citizens cannot be isolated from the dangers of the outside world; trouble is not “always someplace else”. On 9/11, the country transitioned from a dream of unending tranquility at home to a nightmare of forever war. With the return of soldiers from Afghanistan, the US is more distanced from the enemy. But the conflict continues by other means, and without boots on the ground, drone strikes are more likely than ever to claim the lives of civilians as well as terrorist suspects. The US, and the west, cannot be safe at home while insecurity reigns abroad.