An attack like this might not be today, but in 10-20 years? If a country can develop this technology where multiple cities or military bases are completely overwhelmed by a swarm of drones, and all the politicians and generals are taken out by AI-controlled drones, why wouldn't they invest in it? As far as I can tell, unless some sort of localized EMP is developed, I don't know how an attack like this can be stopped. Maybe some sort of RF beam but I'm sure the devices can be shielded from that.

It's much more effective than a cruise missile because you can just blow up weak points on bridges, buildings, take out entire military bases, etc. Even 1000 of these drones would be extremely effective but 1 million would be devastating.