As Vali Nasr and others have argued persuasively, the U.S. held the greatest amount of leverage vis-Ã -vis the Taliban during and immediately after the troop surge. That being said, it is wholly untrue that the U.S. would have no ability to influence the Taliban after withdrawing from the country. There is at least one carrot the U.S. could offer the Taliban: greater autonomy from Pakistan. Indeed, although seldom acknowledged, the U.S. and the Taliban have an overlapping interest in limiting Pakistani influence in Afghanistan.For Washington, this interest derives from Islamabad's support for fundamentalist groups that attack the U.S. and its allies, as well as its increasingly cozy relationship with China. Although the Taliban rose to power in no small part because of Pakistani assistance, all reports suggest that the group has soured on its patrons during its stay in Quetta, as a result of being repeatedly treated like a pawn by Pakistan's powerful security and intelligence forces.
While dependent on Pakistan's hospitality, the Taliban have had to live with this humiliation. If they returned to power in Afghanistan, one of their first goals would almost certainly be to reduce this dependency on Islamabad. The most practical way of doing this would be to establish some sort of relationship with other powers, which could then be used to counterbalance Pakistani influence. The Western powers (possibly with regional buy-in) could offer to play this role if the Taliban were to respect their most basic interests in Afghanistan--principally, preventing terrorist groups from operating inside Afghanistan, and preferably some degree of respect for the human rights of Afghan citizens, especially women and minorities.
Ultimately, however, the West could never be sure that the Taliban would accept these terms and, even if they did, would have to consider that the group could later renege on them. Fortunately, the U.S. and its allies wouldn't be completely dependent on the Taliban upholding the bargain as they'd retain a trump card to prevent Afghanistan from returning to the country of the 1990s and early 2000s. This trump card would simply be to take unilateral action to prevent al-Qaeda from operating freely in Afghanistan.
Having just spent over a decade inside the country, it's reasonable to assume the U.S. has built up greater local networks than it had in the 1990s. These could be used for intelligence purposes and as hired guns against the terrorists. The fact that since 9/11 the U.S. has acquired a global fleet of armed predator drones would also aide in its fight against global jihadists seeking to use Afghanistan as a base to conduct foreign attacks. In fact, this policy would not be unlike the one being pursued in Pakistan, albeit local collaboration would not come from the national government as it often does in Pakistan, but rather from various non-governmental Afghan groups.
In many ways, it would be preferable for the U.S. to conduct covert action (including drone strikes) against terrorists inside a Taliban-ruled Afghanistan rather than Pakistan. To begin with, the U.S. has stronger local networks inside Afghanistan than it does in the foothills of Pakistan. Moreover, the Taliban would be far less capable of defending Afghanistan's sovereignty than is true of the government along its eastern border. Similarly, destabilizing Afghanistan through drone strikes would also be inherently less risky than destabilizing a much more populous and nuclear-armed Pakistan.
To be sure, this scenario is far from an ideal settlement to the Afghan conflict, which has become the longest war in American history. And certainly the prospect of the Taliban returning to power in Afghanistan would greatly concern many regional states, including U.S. partners like India. But at this juncture there are no ideal conclusions to the Afghan conflict available, and not actively trying to prevent the Taliban from returning to power is hardly an unacceptable one.