Images of aeroplanes smashing into the World Trade Centre, of people jumping to their deaths, of collapsing towers and waves of choking dust filling the New York streets will forever live in my memory.

That day the world stood shoulder to shoulder with the US.

George W Bush, the recently elected president, now had the opportunity to shape the ‘new world order’ his father had promised after the collapse of the Soviet Union.

Bush needed to do two things: allay the fears and anger of Americans and bring the terrorists to justice. In so doing he and his government would earn respect and recover the moral authority lost by the US over much of the post-war period.

Standing in the rubble of ground zero, addressing exhausted emergency workers, however, Bush chose a different strategy. He vowed revenge and a war on terror. The bombing, invasion and occupation of Afghanistan in pursuit of one man, Osama Bin Laden, leader of al-Qaeda, was underway.

But what was the strategy beyond the ousting of the Taliban and Bin Laden’s capture? Nation building? Ending terror attacks on the west?

Twenty years on and it’s clear there was no strategy beyond revenge.

In that time US administrations, both republican and democrat, have ignored international law, torturing suspects and denying them basic human rights under the Geneva Conventions.

Of the nearly 800 detainees held at Guantanamo Bay, only two have been charged and found guilty. Half a million Afghans are dead and injured. Two million are refugees.

Having failed to catch Bin Laden early on, the Bush administration escalated the conflict. Their next target was Saddam Hussein, an erstwhile ally who had turned against the US by invading Kuwait.

Claims that Saddam was in cahoots with al-Qaeda and had an arsenal of weapons of mass destruction were manifest lies, but that didn’t stop the shock and awe offensive of 2003.

By 2006 the Taliban had started to regroup in Afghanistan. In Iraq al-Qaeda took advantage of the power vacuum to muster support against the occupiers; the very same al-Qaeda which would go on to to establish the Islamic State across the Levant.

Still the Americans, assisted by the UK government, pressed on, committing more troops and money to corrupt regimes who neither served their foreign masters nor their impoverished citizens.

Twenty years on, the US intervention in Afghanistan has come to an end with yet another ignominious defeat and yet more harrowing images of desperate people clinging to aircraft.

I leave you with the words of GR Gleig, a British army chaplain, who wrote that this was “a war begun for no wise purpose…brought to a close after suffering and defeat…Not one benefit, political or military, was acquired with this war.”

Only Gleig was writing in 1843 just after the first Anglo-Afghan war, known ever since as the Disaster in Afghanistan.

George Bush’s biggest mistake was to ignore history. And that is a lesson for all political leaders.

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