Before the Second World War, HMS Regulus reportedly encountered the newly commissioned carrier Hiryū while patrolling near Japan. The submarine followed the carrier through Japan's annual naval exercise without being detected and returned with photographs. In the hypothetical continuation, another British submarine might observe Japanese carriers assembling and follow their departure toward Hawaii. A radio warning could then give American fighters time to prepare for the approaching attack. The actual reconnaissance anecdote and the imagined warning of Pearl Harbor remain separate parts of the comparison.
Notable Point · 1:28:12-1:29:41
HMS Regulus shadows Hiryū during a Japanese fleet exercise
A prewar British submarine reportedly followed the newly commissioned carrier Hiryū through a Japanese exercise, illustrating the intelligence value of forward submarine patrols.