Confronted with strong public support for a substantial drawdown of U.S. troops in Afghanistan, the most die-hard defenders of the mission have attempted to make a convincing case for staying. Their arguments rest on a few core assumptions about what will happen if U.S. forces are withdrawn “prematurely” (an odd concept, considering that U.S. forces have been in Afghanistan for nearly 10 years); and on the likelihood that a continued large-scale presence will achieve the Obama administration’s stated objectives.

Both sets of assumptions are flawed. The relative certainty about the horrible events that will transpire in Afghanistan and beyond after foreign troops are withdrawn is completely unwarranted, as are the claims that the United States will inevitably be victimized by another attack, and forced to reenter Afghanistan in large numbers again in the future. The confidence that staying longer will prove instrumental to success is equally misplaced. Most nation-building missions fail, and the conditions that have prevailed in the few successes (e.g. Germany and Japan) don’t exist in Afghanistan. Leaving U.S. troops in the country for another five or ten or twenty years is unlikely to change that. It is certain, however, that a continuation of the current posture in the country will further drain our treasury, tax our troops, and render us less capable of addressing other threats that emanate from other places.

On the first point, here is what Kimberly and Fred Kagan warn will happen if the Obama administration announces its intention to withdraw “all surge forces from Afghanistan in 2012”:

the war will likely be lost. Al Qaeda, Lashkar-e-Taiba, and other global terrorist groups will almost certainly re-establish sanctuaries in Afghanistan. The Afghan state would likely collapse and the country would descend into ethnic civil war.

The Kagans would like us to believe that they have a crystal ball that allows them to predict the future. But no one has any idea what will transpire in Afghanistan after a complete American withdrawal, and it is even less clear what a reduction of up to 30,000 troops (less than one third the total number in country) would produce. Further, even if it were reasonable to speculate that Afghanistan post-2014 will look a lot like Afghanistan pre-2001—i.e. mired in “ethnic civil war”—that doesn’t mean that it “would directly increase the threat to the American homeland” as the Kagans claim.

Finally, if the Taliban somehow manages to reestablish control over a sufficient portion of Afghanistan, and then chooses (foolishly and recklessly) to allow al-Qaeda to reestablish a presence there, we have ways of dealing with the threats short of stationing 100,000 troops in the country at a cost of over $100 billion every year. After all, al-Qaeda’s capabilities had been severely degraded even before U.S. forces killed Osama bin Laden in Pakistan. It is simply false to claim, as the Kagans do, that a withdrawal—any withdrawal—of U.S. forces, either this year or next, will leave us with just two choices in the future: either to “accept likely attacks on the U.S. homeland or to intervene militarily once again—at a much higher price than we could hope to save now.”

This isn’t just unwarranted speculation; it is fearmongering of the worst sort. It represents a desperate attempt to convince the American people that there is no choice but to continue on our current course.

But the claims that current course is likely to yield success are equally problematic. Indeed, it is curious that the same people who are so pessimistic about our prospects for preventing another attack here in the United States are so wildly optimistic that we can succeed in fashioning a functioning nation-state in Afghanistan. In fairness, the Kagans' article was published before the release of a report by the Senate Foreign Relations Committee showing that our nation-building efforts have not merely failed, but have arguably made many of the problems in Afghanistan even worse. Incoming ambassador Ryan Crocker didn't help the Kagans' case during his confirmation hearing yesterday (the soundbite that success is not "impossible" speaks volumes). But they and others who share their peculiar faith in this mission surely know (or, at least, they should) that most nation-building missions fail, and that the conditions for success have never existed in Afghanistan, and are unlikely to magically materialize in the next 12 to 18 months.

The Afghan state is so poor and fractured that the massive amounts of western assistance plowed into the country so far have merely worsened already rampant corruption and deepened a sense of dependency. If anything, President Obama’s decision to dramatically expand the war in 2009 and to increase the volume of aid flowing into Afghanistan likely exacerbated a pernicious cycle. It is hard to understand how a decision to leave 100,000 U.S. troops in Afghanistan indefinitely, and to continue such aid, is likely to change anything.

* With due respect to Simon Schama.