Japan belonged high on the Royal Navy's prewar risk register because British and Japanese forces were already confronting one another's interests in Asia. Tension sharpened during the 1930s as Japan operated in China and the former Anglo-Japanese partnership broke down. Britain used the Washington Naval Treaty and American pressure as a convenient public explanation for not renewing the alliance, which softened the admission that London objected to Japan's political direction. That diplomatic maneuver may also have worsened Japanese hostility toward the United States. In this setting, designing merchant ships for rapid amphibious conversion against a possible Japanese war becomes strategically plausible even though no direct planning document is offered.
Notable Point · 25:22-27:55
Why Japan framed British prewar amphibious planning
Deteriorating relations with Japan made a Pacific amphibious requirement more immediate to British planners than a simple focus on Germany suggests.