I think it is stretching to say many on the left opposed Afghanistan from the start. Definitely more on the left than the right, but W had like an 80% approval rating at the start of the war. Iraq is a different story, though. I was pretty against Iraq, but didn't initially have a problem with Afghanistan, especially as originally pitched.
Depends how you define 'left' I suppose.
Most of the leftists I knew were predicting that US imperialism would meet disaster in Afghanistan.
It's true that I didn't really see it the same way, having moved a long way towards the centre myself (and the Taliban are ideologically a very long way from some socialist national liberation movement). To be honest I kind of felt if the Americans wanted to get into that mess it was up to them, and if the mad misogynists of the Taliban were dislodged from power in the process, so much the better. But I thought the UK should stay out of it.
What the purpose was of staying there for 20 years, I particualrly don't understand. I thought they'd just blow some stuff up till the American public was satisfied they'd gotten revenge for 911, then get out. I wonder if part of the motivation for invading Iraq was simply that there wasn't enough of substance in Afghanistan to blow up?
I don't really understand what the motivation for any of it was, not even Iraq. With Iraq some claim it was about 'oil', or removing a threat to Israel, but it seems like it was more about some kind of weird psycho-political thing, that the Western Elites needed a 'crusade', something violent and interminable, to somehow stamp their 'will' on the world in some strange Neitzchean psychodrama. Maybe it was just a reaction to all those decades of being frustratingly constrained by the Cold War, stopping them flexing their military muscles?
In any case the neocon movement seems to have failed at least as comprehensively as communism did. I really hope it's gone for good now.