It may also have been unnecessary. If the Taleban regime in Kabul was not told beforehand about al-Qaeda's 9/11 attacks on the United States, then Osama bin Laden betrayed his hosts. Maybe they could have been persuaded to hand him and his men over by a mixture of threats and bribes.
So if Nato is now conceding that the Taleban cannot be crushed by military force, then why is it going to keep its troops in Afghanistan for another two-and-a-half years before acting on that conclusion? Some of them will die as a result of that decision, and quite a few Afghans will be killed because of it, too. Apart from temporarily saving the face of various Western governments, what purpose will their deaths serve?
Nato's argument is that another two years will leave the Afghan army in a better position to defend the US-installed government of Hamid Karzai after Western troops leave, but there is absolutely no evidence that it is true. Indeed, of the 150-odd Western troops killed in Afghanistan so far this year, twenty were killed by the Afghan troops that Nato is supposed to be training for this role.
The "Afghan National Army" is not fit for purpose, and the outcome after Nato troops leave will probably be the same whether they all go home this year or stay until 2014. So what is that probable outcome?
Karzai may not fall immediately: the $4 billion a year that Nato is promising to pay for the maintenance of his army will probably preserve the status quo for two or three years. But no more: it is most unlikely that the subsidy will be extended when it comes up for review in 2018.
That's the way the Vietnam war ended. The last US troops left South Vietnam in 1973, but the regime they left behind survived until Congress cut off the flow of military aid in 1975. It happened exactly the same way when the Russians left Afghanistan in 1989: the regime they had supported lasted three more years, until the flow of funds was cut off after the old Soviet Union collapsed at the end of 1991.
The same thing will almost certainly happen this time. Even the $4 billion that NATO is now pledging will only pay for an Afghan army two-thirds of its currently planned size. When the external funding ends, the roof will probably fall in on Karzai's regime.
The Taleban will doubtless keep control of the Pashtun-speaking provinces where they recruit most of their fighters. (For all Nato's efforts, they never really lost it.) The Afghan National Army will probably disintegrate and be replaced by the separate but allied Tajik, Hazara and Uzbek ethnic militias that held the north of the country before 9/11. They may be able to hold it again.
In other words, the likeliest outcome is a reversion to the pre-9/11 distribution of power in Afghanistan, perhaps with the Taleban in control of Kabul, perhaps not. That's not a wonderful outcome, but it's not such a terrible one either.
Gwynne Dyer is a London-based independent journalist whose articles are published in 45 countries.