It seems to me that there is no general rule, here or elsewhere.
But the crucial issue may be identifying the enemy and knowing what he wants in each case (its not a she in Afghanistan).
I have no inside information, but I have a number of leftist pakhtoon friends and they make two points that may appear contradictory.
1. There is a jihadist core in pakistan (not in "sanctuaries" alone, but in the govt, in the Islamist parties, especially in the intelligence agencies) and they are fleecing the Americans while ruthlessly protecting their minimum interests (even if that means sacrificing a lot of foot-soldiers).
2. Pakistan is not a very strong state. No one in the ruling elite actually wants to try to fight off an unhappy America or even to survive without monthly handouts. The bluff works only because America lets it work.
If both points are correct (and I vacillate between believing both and being skeptical) then the problem is never going to reach some imaginary worst-case scenario. Point two trumps point one.
Having said that, in the interest of full disclosure, I would add that as an American I dont see what all of this has to do with any imaginary "war on terror". There is no enemy out there that needs this response. Even if this is being done for Israel or for oil, its a waste of effort. Israel has problems closer to home and is a big boy and should handle its own problems. The oil comes from the gulf, not from Afghanistan. Much smaller investments in carrots and sticks and other measures would keep Islamist terrorism in manageable bounds in the west.
All this is only meaningful if the US is worldcop and wants to do some social engineering in a bad neighborhood. Otherwise, why bother? Let them kill each other or make peace or make love or whatever. India is at risk, but they think they are big boys too. Besides, if they could handle kashmir in the nineties, they should be able to handle X or Y in the future as well.