But drone strikes often aren't as accurate as their proponents claim. The Bureau of Investigative Journalism estimates that the 415 drone strikes launched in Pakistan since 2004 have killed between 2,449 and 3,949 people. Of these, between 400 and 1,000 have been civilians. Micah Zenko, an expert in drone warfare at the Council on Foreign Relations, told The New York Times that of the eight U.S. citizens killed by American drone attacks, seven were killed unintentionally—including Warren Weinstein, one of the two hostages who died in the January strikes.

Technology has improved in the decade-plus since the American drone war began. A second-generation armed drone called the Reaper, for example, can locate a target to a single room within a house. But drone strikes are only as accurate as the intelligence they're based on. The Obama administration admitted on Thursday that it did not know Weinstein or Giovanni Lo Porto, the Italian aid worker killed alongside him, were in the compound when it was struck in mid-January, despite months of video surveillance. The White House also did not know that Adam Gadahn, a California-born spokesman for al-Qaeda, was present when he died in a separate January drone attack.

Gathering reliable intelligence in the tribal areas of Pakistan, a rugged, remote area largely beyond the federal government's control, remains a formidable challenge. According to a report in Bloomberg, the U.S. is reluctant to send agents to the area. The nature of al-Qaeda itself—a fluid, stateless organization lacking a clear bureaucracy—makes it difficult for foreign spies to infiltrate. Terror groups have also learned how to minimize use of technology that could betray their location and activities.

Since embracing drone warfare in 2009, the Obama administration has shown a willingness to strike known terrorist targets—even when the possibility of collateral damage exists. The attacks on the compound holding Weinstein and Lo Porto—as well as that on the one holding Gadahn—were deliberate strikes on al-Qaeda targets, little different from hundreds of others launched since 2004. Dan Benjamin, a former State Department officer now at Dartmouth College, told Bloomberg's Josh Rogin that the accidental killing of hostages had simply been a matter of time.

"If you are taking kinetic action against terrorist targets for years and years on end, the law of averages is going to catch up with you and something terrible like this is going to happen," he said.

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