One of the most critical items required to run the U.S. and NATO campaign in Afghanistan is jet fuel. Some 30,000 tons a month has to be brought in. That's over 50 trucks a day going through the Khyber pass carrying just jet fuel. These vehicles supply not just military needs, but also civil aviation. In fact, most of the fuel goes to power non-combat aircraft (military and commercial transports). Nearly as many trucks carry other types of fuel (gasoline, diesel).
There are four Pakistani fuel companies that supply most of this fuel, and it's a very lucrative business. Heavy security for the fuel trucks, as well as bribes (for bandits and Taliban) insure that attacks are minimal. Meanwhile, NATO and the U.S. are starting to bring in fuel via the Russian railroads (which end at a rail yard in northern Afghanistan and still have to be trucked south).