Two events this week framed the argument over the use of drones in Pakistan and Afghanistan. Al-Qaida confirmed that one of its founding members — a man named Mustafa Abu al-Yazid — was killed by a drone attack inside Pakistan. U.S. officials called his death a major setback for al-Qaida.

The second event was the U.S. military's investigation into the deaths of 23 Afghan civilians in February. It concluded that a team operating a surveillance drone made a mistake and identified a convoy of vehicles full of women and children as an insurgent target.

The two episodes bring into stark relief the most basic question for U.S. policymakers: Do drone attacks work?

No one will argue that the technology is seductive. Drones circle silently overhead and can watch a target for hours at a time without being detected. Then, they can strike without warning. The CIA and the U.S. military both have drone programs.

"On the one hand, they are powerfully effective at eradicating our enemies," said Samuel J. Rascoff, a law professor at New York University and former intelligence chief at the New York Police Department.

The problem, he notes, is that the killings can alienate the people of Pakistan and Afghanistan.

"On the other hand, they might simultaneously be powerful tools at motivating our enemies," Rascoff said. "So from a counterterrorism standpoint, they are very effective; from a counterinsurgency standpoint, they raise lots of questions."

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