More than 400 large U.S. military drones have crashed in major accidents around the world since 2001, a record of calamity that exposes the potential dangers of throwing open American skies to drone traffic, according to a year-long Washington Post investigation. Since the outbreak of the wars in Afghanistan and Iraq, military drones have malfunctioned in myriad ways, plummeting from the sky because of mechanical breakdowns, human error, bad weather and other reasons, according to more than 50,000 pages of accident investigation reports and other records obtained by The Post under the Freedom of Information Act.