The Pentagon acknowledged on Friday that a U.S. drone strike in Afghanistan on Aug. 29 that officials said was necessary to prevent an attack on American troops was a tragic mistake that killed 10 civilians, including seven children.The explosives the military claimed were loaded in the trunk of a white Toyota sedan struck by the drone’s Hellfire missile were most likely water bottles, and a secondary explosion in the courtyard in a densely populated Kabul neighborhood where the attack took place was probably a propane or gas tank, officials said. In short, the car posed no threat at all, investigators...