Over at Bellum, Mark Rice argues for staying in Afghanistan to preserve US credibility. Aside from the fact that the loss of more life in defence of credibility is morally unacceptable, the comparison with Vietnam (denied in the article but clearly being used as an example) is a false one. He uses credibility as a justification for the his definition of the goal of NATO intervention - the conflict of liberal democracy against Islamism.
There is no doubt that the Taliban are Islamists. But that was not the goal of NATO intervention. It was to destroy the central command of al Qaeda. The goal has been accomplished. To now move the goalposts in an increasingly unpopular war to support a corrupt government (much like Vietnam, by the way) is intrinsically misleading. This is not a war for the hearts and minds of millions of people as he asserts. The argument that everyone is watching to see how NATO handles the conflict - in particular Russia - is to assume that NATO has a strategic vision of its future. It does not.
Many NATO allies have a marginal interest regarding the Afghan mission and there are profound differences over the appropriate means to address the threat of transnational terrorism in general. Europeans do not think that the threat of jihadism should be countered in the Middle East and South Asia with major commitments of troops, but rather at home using domestic law enforcement or, as Vice President Biden advocated, handled abroad with clandestine operations.
Nevertheless, the US still has both a motivation to bring the senior leadership of al Qaeda to justice and a strategic interest in leaving Afghanistan with a government capable of preventing the country from devolving into a terrorist safe haven. The problem is the overcommitment to combating terrorism in this manner at the cost of threats elsewhere and with so much of its resources and political capital invested, it is reluctant to withdraw. This does not mean that it should stay - and most certainly not to retain some ephemeral credibility. Europeans, after all, have essentially already left.