The wiping-out-towns problem is already solved by aerial bombardment or poison gas. Notice that full-size drones are already being used for targeted killing. Swarms of fly-sized assassins, with some means of identifying targets, are a logical development.
Fighter pilots who have survived being locked on by homing surface-to-air missiles describe the experience as "soul-destroying." I suspect that unleashing a horde of dog-sized robots, each of which picks an enemy soldier at random and hunts him specifically, might turn a defensive trench into chaos as soon as the defenders realized what was happening.
The robotic battlefield might come about sooner than we expect, as a result of human psychology.