After the terrorist attacks on Sept. 11, 2001, the United States government and its citizens were largely united in their desire to retaliate. But the question was: Against whom? Enter the Authorization for Use of Military Force (AUMF), passed by Congress on Sept. 18, 2001. Basically, the law gave the president permission to go after whatever country, organization or person was responsible for the attacks so they couldn't do it again.
Initially, the president used the authorization to strike al-Qaida, the group that carried out the attacks, and the Taliban in Afghanistan, who harbored them. Since then, however, AUMF has been used to justify everything from the Guantanamo Bay detention center to — you guessed it — drone strikes against suspected terrorists [source: Currier].
The first armed drone mission was carried out in Afghanistan on Oct. 7, 2001, and drones have been used during traditional military operations in Libya, Iraq and Afghanistan ever since. The more controversial strikes, however, have occurred in countries with which the United States isn't actually at war. These include Pakistan, which U.S. drones first hit in 2004, in addition to Yemen and Somalia, where strikes have been going on since 2011 (with the exception of one strike in Yemen in 2002) [source: Matthews].
Everything about drone strikes is a big secret. In fact, the Obama administration didn't even formally acknowledge the program until April 2012, and it rarely comments on specific strikes [source: Miller]. This secrecy makes it difficult to know for certain how many strikes there have been and how many people have been killed as a result.
The best data puts the number of strikes in Pakistan somewhere between 396 and 415, with 2,232 to 3,949 killed as of May 2015. Of these, somewhere between 262 and 962 were civilians [sources: New America, BIJ]. Yemen has experienced between 95 and 206 strikes, which killed between 65 and 158 civilians out of 447 to 1,117 total killed. Finally, nine to 13 strikes in Somalia killed a total of 40 to 105 people. It's estimated that up to five of those were civilians [source: BIJ].