The U.S. military are in Pakistan because it enables access into Afghanistan, home of the Taliban and Al Qaeda training camps. Afghanistan also has a route for a natural gas pipeline from Iran to India and would provide Iran a means to pay for nuclear weapons. Natural gas sales would offer a hard currency supply that will support the economically troubled regime in Iran, now with 16% inflation and 24% unemployment. Iran's oil is nearly out but they have significant reserves of natural gas. Exporting it by ship is dangerous and expensive and nations generally avoid getting ports due to historical safety concerns. Natural gas is usually exported via pipeline because they're safer to operate and offer a secure means for trade.
The U.S. started a war in Afghanistan to contain and eradicate terrorist training camps run by the Taliban. Pundits from the international energy field say it was to covertly blow up the gas pipeline and prevent Iran from getting money from India. Iran states they want the money so they can use nuclear power to advance their national interests in the Persian Gulf and Israel. Iran has also stated it is opposed to ruling parties in nearly every nation in the Gulf except Syria and by extension Lebanon, which is heavily influenced by its neighbor Syria. Once Iran gains the infrastructure to manufacture and maintain nuclear weapons, they can negiotiate favorable conditions for their own national interests. Speculation by international experts that Israel will attack Iran's nuclear facilities, similar to their attack in 1981 on Iraq's nuclear power plant near Baghdad. The Saudi Arabian government has also expressed concern over a nuclear armed Iran.
Present U.S. military involvement in Afghanistan and Pakistan over Iran's pipeline through the region has been costly. Ongoing operating costs requires payment of $1B/month to various persons, departments and associated agencies in Pakistan, which pundits refer to as "bribes" to allow U.S. military operations in the area. "At some point the dollar will devaluate until it is no longer a sufficient means of payment and Pakistan is going to demand weapons we don't want to give them, or gold. And then its over," state international experts familiar with the situation.
Some argue that there are minerals in Afghanistan that justify Western national interests. Others state that there's humanitarian reasons to convince the population to support a modernization of the region. Primary business in Afghanistan is growing opium poppies for export, fuelling world heroin demand and associated crimes.
Pundits state that a U.S. withdrawal from South Asia could start a bigger war in the area. They also point out that the last decade hunting Osama Bin Laden turned him up in Pakistan, an alleged "ally" in the war on terror since 9/11/01.
(edited in the AP style just to see if I could pull it off)