Royal Navy submarines scouting inside Japanese waters

Royal Navy submarines based in Hong Kong conducted covert reconnaissance patrols in and around Japanese home waters before the Pacific War. Regulus observed carriers and a Combined Fleet exercise, while the wider effort reached narrow channels and bays where major units could be photographed. Japanese forces later found some of the resulting imagery in Singapore after its fall. The operations foreshadowed the intelligence role that submarines would perform after 1945. Their termination when British submarines left the Far East in 1940 removed a collection method that might have corrected later errors about Japanese warships.