There are reasons that Al Qaeda might be reluctant to relocate. The protective, rugged terrain of Pakistan’s Waziristan region and the group’s ties with the tribes there offer advantages the terrorist network would struggle to replicate in other places such as Somalia or Yemen.

Pakistan has served as such an effective haven that after eight years, the trail for Bin Laden “is not just cold, it’s frozen over,” Riedel said.

Moving across the border would expose Al Qaeda operatives to risks of detection by satellites, CIA drones or spies on the ground. And even if the U.S. was to withdraw forces from Afghanistan, America is likely to have a substantial military presence there — and freedom to operate, unlike in Pakistan — for years to come…

But there are concerns that Afghanistan could become a safety valve for Al Qaeda as the pressure in Pakistan mounts. Spilling across the border would provide an escape not from the Predator aircraft, but from the spies guiding the drones to their targets.

Hoffman and others argue that intelligence on Al Qaeda could wither if U.S. forces are not in position to protect Afghan villagers and persuade them to give up suspected terrorists.