It is powered into flight by a rocket engine. It can fly a distance equal to the width of China. It has a stealthy design and is capable of carrying missiles that can hit enemy targets far beyond its visual range. But what really distinguishes the Air Force’s pilotless XQ-58A Valkyrie experimental aircraft is that it is run by artificial intelligence, putting it at the forefront of efforts by the U.S. military to harness the capacities of an emerging technology whose vast potential benefits are tempered by deep concerns about how much autonomy to grant to a lethal weapon. Essentially...