Juan Cole, Informed Comment: A common explanation for the US presence in Afghanistan is Washington's interest in Central Asian fuel sources -- natural gas in Turkmenistan and Uzbekistan and petroleum in Kazakhstan. The idea of Zalmay Khalizad and others was to bring a gas pipeline down through Afghanistan and Pakistan to energy-hungry India. Turkmenistan became independent of Moscow in 1991, making the project plausible. For this reason some on the political Right in the US actually supported the Taliban as a force for law and order.
If that was the plan, it has failed. Instead, China has landed the big bid to develop a major gas field in Turkmenistan, along with a pipeline to Beijing. Turkmenistan had strongly considered piping the gas to Moscow instead, but developed conflicts with Gazprom.
So the US is bogged down in an Afghanistan quagmire, and China is running off with the big regional prize...
I'm not sure very many politicians in Washington were ever really so interested in the gas pipeline. For someone like then Secretary of Defense Donald Rumsfeld, making Afghanistan a US base may have aimed at surrounding and weakening Russia and keeping it from reemerging as a peer (a la the attempted push of NATO in places like Georgia).
Some US leaders, however, were pushing for it. In recent years a Turkmenistan pipeline was seen as a way of forestalling India from breaking the embargo on Iran. And I remember that in fall 2001, when congressmen asked Colin Powell how the Afghanistan war would be paid for, he replied that the region is rich in resources...
In any case the Chinese just demonstrated that you don't need war to get resources. Avoid costly adventurism and grow your economy like hell, and it all falls into your lap.