Why equal submarine limits would be hard to police

An equal submarine-tonnage formula would probably leave British and American procurement broadly intact while creating a serious enforcement problem and greater Japanese pressure to exploit the limit.

Question

How would an equal submarine-tonnage limit have changed naval procurement?

Short answer

Britain and the United States would change little, while Japan would push the ceiling and enforcement would remain exceptionally difficult.

Notes

The Royal Navy and US Navy are expected to buy roughly the submarine forces they historically wanted. Japan, by contrast, would have strong incentives to press the allowance to its maximum. The central difficulty is verification, because submarine tonnage is unusually hard for treaty partners to police. The answer exposes the gap between a neat numerical rule and the practical problem of making every signatory comply.