Nearly eight years ago, U.S. forces first entered Afghanistan to pursue the al Qaeda terrorists who plotted the Sept. 11 terror attacks. American troops are still there today, along with thousands of NATO forces. Under a new strategy crafted by the Obama administration, military leaders are trying to deny terrorists a permanent foothold in the impoverished Central Asian country and in neighboring, nuclear-armed Pakistan, whose western border region has become a sanctuary for Taliban and al Qaeda forces. The Afghanistan-Pakistan conflict — “Af-Pak” in diplomatic parlance — poses huge challenges ranging from rampant corruption within Afghanistan's police forces to a multibillion-dollar opium economy that funds the insurgency. But those problems pale in comparison with the ultimate nightmare scenario: Pakistan's nuclear weapons falling into the hands of terrorists, which foreign-policy experts say has become a real possibility.