Pardon my ignorance, but I thought that mass use of drones was part of the Gulf War.
Getting the Iraqis to expend SAMs or light up their radars to target drones pretending to be bombers, while following up with actual bombers, was apparently very successful.
In fact the drones fooled the Iraqi defenses so well that they thought they were doing a lot of damage, shooting down hundreds of planes. Instead they were blowing up drones that cost less than the SAM used to twat them.
Of course if you want to consider a missile or a V2 to be an autonomous drone, then we've had killer drones for quite some time.