Despite Mr. Brennan’s protestations, an overwhelming reliance on killing terrorism suspects, which began in the administration of George W. Bush, has defined the Obama years. Since Mr. Obama took office, the C.I.A. and military have killed about 3,000 people in counterterrorist strikes in Pakistan, Yemen and Somalia, mostly using drones. Only a handful have been caught and brought to this country; an unknown number have been imprisoned by other countries with intelligence and other support from the United States.

This policy on targeted killing, according to experts on counterterrorism inside and outside the government, is shaped by several factors: the availability of a weapon that does not risk American casualties; the resistance of the authorities in Pakistan and Yemen to even brief incursions by American troops; and the decreasing urgency of interrogation at a time when the terrorist threat has diminished and the United States has deep intelligence on its enemies.

Though no official will publicly acknowledge it, the bottom line is clear: killing is more convenient than capture for both the United States and the foreign countries where the strikes occur.

The drone strikes have become unpopular abroad; in a Pew Research Center poll last year, just 17 percent of Pakistanis supported them against leaders of extremist groups. And domestic critics have attacked from two different directions: Some Republicans in Congress accuse Mr. Obama of adopting a de facto kill preference because he shut down the C.I.A.’s overseas prisons and does not want to send more detainees to Guantánamo Bay, Cuba. Human rights advocates argue that some drone strikes have amounted to extrajudicial killings, the execution without trial of people suspected of being militants whose identities American officials often do not know and who sometimes pose little threat to the United States.