Enlarge the Picture
And see why the West now has three options;
(i) Drag the Iranians and Pakistanis into different bits of Afghanistan, deny it to Russia and China, and push them into dependence on an alliance with the West
(ii) Stay in Afghanistan and do well enough to convince the Russians that we haven't lost and get a deal with them, based on a rapprochement over the eastern European missile shield.
(iii) Pressure, finagle, manipulate or seduce the Iranians into a deal
Alternatively, we could depend upon the felicity of our high purpose and the charity of the global community.
The Georgia War already dealt a blow to western schemes. Removing the missile shield may well just make the Russians think they can pressurise the east with higher gas charges, not work with us to lower ours.
Europe needs North Africa--as a camp to concentrate migrants and as a source of gas-- plus at least one of these routes, but America must deny as many as possible of the routes to China, Iran and Russia. If it does not its natural competitors will wax as it wanes. It could conceivably sit things out in bases in central Asia, but only if someone else takes the offensive in Afghanistan.
In addition, if the west runs from Afghanistan now, sooner or later what takes over will spill into the Balkans--look at where those pipes end, incidentally--and worsen the problems that the west has with alienated, militarised, and islamicized sections of the Muslim youth and middle-aged.
I found the map on the leftwrite blog, which has a slightly different take from me, (and which seems somewhat Leninist) here. Though (I know) an increasingly conservative catholic, I thought that I should nevertheless acknowledge Leftwrite's analysis and utility