I n a small workshop in England’s East Midlands, engineers at the British startup Skycutter are designing weapons for Ukraine . A row of 3D printers make the fuselage for interceptor drones, while parts such as motors and navigation chips are slotted together by hand. The same process happens hundreds of thousands of times a month in partner Ukrainian factories. The swarms of cheap, deadly and often autonomous drones deployed in that war have already changed combat completely. Troops far behind the frontline must move constantly to avoid attack from the air, travelling along netted tunnels and landscapes crisscrossed by fibre optic cables used to steer drones past radio jamming. Cities are terrorised by guided missiles that are cheaper and therefore more widely used than those that came before.…