Grenier told DW that he feared from the start "we would ultimately end up in a kind of situation in which we currently find ourselves. A never-ending struggle where there is no way of defeating our antagonists, but where they too are incapable of winning and you just have a protracted struggle."

Asked about the failures of the military intervention that made Afghans view the US and NATO forces as occupiers rather than liberators, Grenier said: "One of the frustrations for those of us dealing with people back at our own headquarters both at CIA and at the Pentagon in particular was, that the people we were dealing with really did not understand very much about Afghanistan or about the dynamics between al-Qaeda and the Taliban for instance. And they tended to view things in very stark black and white terms."

Grenier added that after the fall of Kandahar in December 2001 "I thought that some effort should have been made to reach out to some elements within the Taliban leadership to provide them with assurance that there would be some future for them in Afghanistan. But there was absolutely no interest from the American side in doing that. At the time we did not realize how important an opportunity it was, or how grave the consequences of our failure to seize that opportunity would ultimately be. I think we missed a major opportunity at that point."

About negotiations with Taliban leader Mullah Omar's deputy, Mullah Osmani, Grenier said: "I tried to convince [Mullah Osmani] to move Mullah Omar aside and to seize power himself to do what needed to be done – both to bring Bin Laden and his key lieutenants to justice but also to save the Taliban movement." Grenier said the negotiations failed because "Mullah Omar had too much control over his own people, and there was no way that Mullah Omar could be convinced himself to turn over Bin Laden."

Looking to the future, the former CIA officer said "it is well within the ability of the international community to provide enough support to the Afghan government that it can preserve itself and can keep most of the country from being overrun by the Taliban. If there is an ultimate peaceful solution in Afghanistan, I think it is going to be a decentralized solution, where you will have people affiliated with the Taliban, who will continue to play an important political role."