The Royal Navy used Japan as a demanding design and planning benchmark for long-range submarines and mine warfare. In this planning logic, a fleet prepared for the Imperial Japanese Navy would also be well placed for many lesser contingencies. The United States offered an even larger theoretical benchmark, but presenting it as the intended enemy would have made funding politically difficult. Japan was both a credible peer threat and a usable case for securing resources.
Notable Point · 34:20-34:58
Japan as the Royal Navy's fundable peer benchmark
Planning against Japan justified Pacific-capable submarines while avoiding the politically implausible case of openly budgeting for war with the United States.