The passage above seems quite obviously true, indeed pretty much common sense.

Yet you don't offer any direct evidence. Moreover, the style of your comment is precisely the reason why political debates aren't encouraged on LW. The problems are:

1. offering only one possible explanation of a selected historical event, ignoring other possible explanations and several important concerns (e.g. you have tacitly assumed that it was feasible to impose any government in Afghanistan without regard to the opinion of the Pashtuns - a dubious assumption in the least)
2. rhetorical questions instead of well formulated arguments ("you may think this account [...] is odd, but if it is odd, is not it odder that ...")
3. implicitly suggesting that the opponent may be biased against new ideas ("or is it just that the entire thing turns conventional wisdom one end over the other")
4. conspirationist-style vocabulary ("conventional wisdom ", "comprehensively contradicts official history") and unnecessary use of political labels ("leftward direction"); this seems to imply that there is some leftist conspiracy to cover the important facts

The points 3 and 4 are mainly a matter of style, but 1 and 2 are more important. Inference (X is odd, therefore Y must be true), where X and Y are neither exhaustive nor enough precisely specified, is a fairly typical ingredient of nowhere-leading frustrating debates. This is what we try to avoid here.