The premise assumes that naval attaches, photographs, and other intelligence would eventually reveal the Yamato class's basic characteristics. The expected discovery concerns 18-inch guns and a speed of 27 knots. The first four planned Lions are considered unlikely to change, while the response among the later ships depends on whether the intelligence arrives in 1940 or 1941. That knowledge creates pressure for a capital ship able to answer it, even though an immediate British redesign remains uncertain. Aircraft and submarines were already changing naval planning, yet decision-makers still wanted a reliable surface answer to a known battleship threat. Governments in the late 1930s had not seen the newer systems prove themselves in wartime and were reluctant to rely on them alone for national security.
Notable Point · 28:43-34:43
Yamato intelligence changes capital-ship planning
If Yamato's 18-inch guns and speed became known before war, British and American planners would have to react before committing fully to the carrier age.