A ROBOT submarine glides stealthily towards the coast before launching dozens of small drones into the air. Individually, these drones can’t do much damage, but as a swarm they are hard to defend against. They can carry explosives or electronic jammers capable of knocking out a radar system or other sensors.

This is the plan for LOCUST, the US Navy’s low-cost, swarming technology, which has just been revealed in Pentagon documents.

The project will probably use Coyote drones made …