Why the fleet-submarine idea awaited better technology

Which navy came closest to developing the fleet-submarine concept before the Second World War?

Several powers explored it, but none had the technological level needed to make the concept fully workable.

More than one power approached the fleet-submarine idea, but none fully realized it before the necessary machinery and operating technology matured. Recognizing a useful operational concept did not mean that a navy possessed the engineering needed to employ it reliably.