Why a secret Kii-class battleship would collapse treaty restraint

What if Japan secretly completed a Kii-class battleship after the Washington Naval Treaty?

The ship would probably be discovered, prompting sanctions or treaty escalation that allowed Britain and the United States to build powerful counterparts.

Keeping a Kii-class battleship secret would be extremely difficult because Britain and the Soviet Union maintained capable human-intelligence networks in Japan. Once exposed, the violation would be too consequential to overlook in the way that some modest cruiser-tonnage evasions were tolerated. Britain and the United States could demand treaty renegotiation, invoke an escalator clause, impose sanctions, or in the most dangerous case consider war. The likely peaceful response would authorize equivalent or greater construction, potentially reviving British fast-battleship studies with 18-inch guns. Japan's temporary advantage would then provoke a much larger industrial counterbuild and undermine the treaty system it had tried to exploit.