I think we should leave Afghanistan as soon as practical, because we aren’t actually serving our most important interests there. Robert Kaplan points out that we are supporting Chinese interests there, however:

[via Beijing’s Afghan Gamble by Robert Kaplan]

“In Afghanistan, American and Chinese interests converge. By exploiting Afghanistan’s metal and mineral reserves, China can provide thousands of Afghans with jobs, thus generating tax revenues to help stabilize a tottering Kabul government. Just as America has a vision of a modestly stable Afghanistan that will no longer be a haven for extremists, China has a vision of Afghanistan as a secure conduit for roads and energy pipelines that will bring natural resources from the Indian Ocean and elsewhere. So if America defeats Al Qaeda and the irreconcilable elements of the Taliban, China’s geopolitical position will be enhanced.

This is not a paradox, since China need not be our future adversary. Indeed, combining forces with China in Afghanistan might even improve the relationship between Washington and Beijing. The problem is that while America is sacrificing its blood and treasure, the Chinese will reap the benefits. The whole direction of America’s military and diplomatic effort is toward an exit strategy, whereas the Chinese hope to stay and profit.

But what if America decides to leave, or to drastically reduce its footprint to a counterterrorism strategy focused mainly on the Afghanistan-Pakistan border? Then another scenario might play out. Kandahar and other areas will most likely fall to the Taliban, creating a truly lawless realm that wrecks China’s plans for an energy and commodities passageway through South Asia.

[…]

Everyone keeps saying that America is not an empire, but our military finds itself in the sort of situation that was mighty familiar to empires like that of ancient Rome and 19th-century Britain: struggling in a far-off corner of the world to exact revenge, to put down the fires of rebellion, and to restore civilized order. Meanwhile, other rising and resurgent powers wait patiently in the wings, free-riding on the public good we offer. This is exactly how an empire declines, by allowing others to take advantage of its own exertions.

Of course, one could make an excellent case that an ignominious withdrawal from Afghanistan is precisely what would lead to our decline, by demoralizing our military, signaling to our friends worldwide that we cannot be counted on and demonstrating that our enemies have greater resolve than we do. That is why we have no choice in Afghanistan but to add troops and continue to fight.

But as much as we hone our counterinsurgency skills and develop assets for the “long war,” history would suggest that over time we can more easily preserve our standing in the world by using naval and air power from a distance when intervening abroad. Afghanistan should be the very last place where we are a land-based meddler, caught up in internal Islamic conflict, helping the strategic ambitions of the Chinese and others.”

So why doesn’t China send in troops? Are we unwilling to fight side by side with them? The lines of supply favor their involvement. Or at least get them to pay for it.

We could could leave this mess in their hands. It’s their backyard, after all.