How other powers might answer the Zepang super battleship

How would other powers react if Japan attempted a 500,000-ton Zepang battleship?

They might wait to see whether Japan could finish it, while beginning the docks and yards needed to answer it if the project succeeded.

A ship of this scale would probably prevent the Washington treaty system from emerging in its familiar form. Other navies could delay laying down a direct rival until Japan proved the project workable, since the attempt might overwhelm Japanese heavy industry. Britain and the United States would still have reason to start the required infrastructure: the estimate allows three to five years for the docks and another three to five for a ship. If Zepang failed, those yards could serve other large construction. If it entered service after five years while rivals had delayed every response, they could face another five years before completing an answer. Political pressure could then force them to build an equally impractical vessel despite its poor fit with their strategy.