Jim, little bit of nitpicking here but if you study insurgencies and guerrilla warfare - Its quite often the opposite. Che Guevara learned this the hard way in Bolivia. These types of movements usually only survive with significant support from the local civilian populace. Not to discount your point because I am sure external players (Pakistan) played a large role as well. The apathy in some parts of the Afghan populace probably contributed to the precipitous fall of multiple provinces.
Ah, we're talking about two different key ingredients that they need to survive. I'm talking about external support in the way Galula talked about, and I wasn't trying to imply that support of the local populace wasn't a critical ingredient. But if you can successfully cut off an insurgency logistically (includes financially) from its foreign patrons then it will soon lack the resources to conduct operations (doesn't mean they will cease operations altogether, just conduct fewer). What comes next is either suppress it and drive it further underground, although it will survive indefinitely from local support, or (to win) not just suppress it but also turn the local population against it.
We and the former Afghan government didn't effectively do either of those things against the Taliban over the last twenty years- neither fully cut it off from outside help (not from lack of a concerted effort either) nor get the population solidly against them (also not from lack of trying).
(Hope that makes sense when I put it that way... I'm pretty sure I'm not explaining anything that you don't already know.)
There will be plenty of books and academic theses written about all this for the next 50+ years. There's occasionally still original material being published about the Vietnam War...