The argument boils down to this: Afghanistan has been part of one empire or another for most of recorded history, and has usually been captured and held without too much difficulty.
But wait, you say. What about the Soviets? What about the Brits? What about Alexander the Great, whose Afghan wound began the decline that killed him?
Well, one argument is that the Soviet retreat was hastened by the arrival of US weapons (although see the comments on the article, which include fascinating and disturbing reflections both by Americans with an historical sense and some Russian nationalists). A better case might be that the USSR under Gorbachev had bigger problems, and ultimately just cut its losses. As for the Brits, yes, they lost big on their first invasion, when an entire army was destroyed. But they won decisively in 1842 and 1880. And Alexander's army managed to hold Afghanistan for 200 years after his death.
Mind you, in the words of the investment houses, past performance is no guarantee of future conquest. A more realistic assessment of Afghan history, and vulnerability, should not be taken as any cheery prediction regarding the current US adventure there. Our situation is uncomfortably close to that of the Soviets -- a spectacular but expensive army, fielded by a nation with economic and political challenges grave enough to make a foreign war seem like an irritating and wasteful distraction.
As longtime readers may recall, we at the Egg were pitiless in our disdain for the Iraq invasion, but have always confessed a sad sense that America's time in Afghanistan is not ill-spent. The people we fight there (although, sadly and crucially, only some of those we actually kill) truly are our mortal enemies, and have proven it repeatedly. The open questions continue to be whether we can defeat them, and whether such a defeat will be worth its many costs -- moral, political and financial.