Juan Cole makes an important point about the odd theory that the "real" motive for the Afghan War was a desire to build a natural gas pipeline from Turkmenistan: A pipeline to convey natural gas from Turkmenistan would be an excellent thing to have. Natural gas is something about which it's hard to say too many good things, it's a hell of a lot cleaner than coal or oil, it's relatively cheap, it can be processed into electricity by very flexible generators, you can make hydrogen fuel out of it, and it has a lot of promise to serve as a "transitional fuel" as we start transitioning away from the oil/coal economy to a non-hydrocarbon one. The good people of Turkmenistan could use more money, as could the Afghans, and the nations of South and East Asia need more energy supplies.
The main trouble with this pipeline is that it hasn't actually been built, and in light of continuing instability it doesn't look like we'll be seeing it very soon.
Now the trouble with natural gas is that transporting it requires liquification to create the oddly named "liquified natural gas" (LNG) and the facilities where you can dock LNG ships, regassify the LNG, and then put it onto pipelines are expensive, ugly, and need to be on the coast, where property values are high and people don't like to see big fuel-processing facilities get constructed. Right now our regas capacity isn't very high, so it's hard to import much LNG and natural gas production in the USA has already peaked even though it would be nice to use more. The current thinking is that we can build the facilities in Mexico and pipe it up from there.