Japanese reconnaissance failed before the carriers met

The Japanese submarine picket line arrived after the American carriers had passed. Earlier signals activity had identified French Frigate Shoals as a flying-boat refuelling point, allowing American patrols to block the planned Kawanishi H8K reconnaissance. The shallow approach forced the refuelling submarines toward the surface, where American warships could see and engage them. Tokyo also warned both Yamamoto and Nagumo about American submarine activity. Yamamoto took precautions and assumed Nagumo would do the same, but Nagumo did not alter his plan. Yamamoto withheld a confirming message to preserve radio silence, leaving their responses uncoordinated. Sending a message by aircraft was another possible channel, although locating and reaching the carrier force created difficulties of its own. Delaying a new code until after preparations for the rushed operation further exposed Japanese plans.