Every decision is ultimately a cost-benefit analysis. I think that the United States government made the decision to go into Afghanistan based on several premises:
1) The United States as a moral purveyor of good has a duty to fight against a moral purveyor of evil/socialism. One such opportunity exists in Afghanistan.
2) The people in Afghanistan are fighting against the Soviet Union to maintain their country and their liberties.
3) The Afghan people will embrace the United States and its way of life because we assisted with their resistance to the Soviet Union.
4) An alliance in Afghanistan gives us a territorial position of advantage analogous to Cuba for the Soviet Union.

While the first is purely a matter of opinion, clearly the second - and critical assumption - turned out to be only partially true. The third one was romanticized and categorically false. (We should have learned from it before attempting to "colonize" Iraq.) The fourth one is true, but gets us to the real cost-benefit analysis question: what is the cost of obtaining and maintaining this strategic position in the world?

Now I join with you in concluding that though there may have been a moral sanction for intervening, the economic costs should have been subject to more explicit public oversight. Has that regional position been worth the billions of dollars - and human lives - which it cost? That's a pretty tough argument to make successfully. But the precipitous pullout from the region has also forced us to confront those very costs of leaving - which are now going to be measured in not only Afghani lives but American lives and a generalized decrease in American moral authority around the globe. How does one put a price on that?

It's also very easy to play armchair quarterback about decisions made decades ago, but the fact remains that once a decision has been made, subsequent decisions have to be made on future considerations rather than the sunk costs of past considerations. That's the part where sometimes such absolute pronouncements tend to appear a little extreme.

It was widely believed at the time that the Afghan government (i.e., the Taliban) harbored Al-Qaeda training camps. That would justify deposing the Taliban and doing a cave-to-cave search for Bin Laden (which wouldn't have found him). Once that was done, we should have been out the door.

Silver lining? The left won't complain about the CO2 emissions.