Changes

In early March, the Japanese postponed their planned seizure of Port Moresby because of previous events. An American carrier raid on Japanese shipping at Lae and Salamaua on the 10th, along with a previously unsuccessful attempt to attack Rabaul on February 20, had demonstrated to Commander in Chief, Fourth Fleet (Admiral Shigeyoshi Inouye), that Japanese were not assured of air superiority in the region. It was not until early May, when Admiral Inouye had three carriers for operations, that the invasion could begin. On May 7-8, the first carrier battle of the war took place in the Coral Sea. Each side had a carrier damaged, while the American lost the carrier USS ''Lexington'' and the Japanese lost the light carrier ''Shoho''. More important the Japanese broke off their invasion attempt. Significantly, American cryptanalysts had provided crucial order of battle and operational communications intelligence to the Allied commanders in the South Pacific.

In early March, the Japanese postponed their planned seizure of Port Moresby because of previous events. An American carrier raid on Japanese shipping at Lae and Salamaua on the 10th, along with a previously unsuccessful attempt to attack Rabaul on February 20, had demonstrated to Commander in Chief, Fourth Fleet (Admiral Shigeyoshi Inouye), that Japanese were not assured of air superiority in the region. It was not until early May, when Admiral Inouye had three carriers for operations, that the invasion could begin. On May 7-8, the first carrier battle of the war took place in the Coral Sea. Each side had a carrier damaged, while the American lost the carrier USS ''Lexington'' and the Japanese lost the light carrier ''Shoho''. More important the Japanese broke off their invasion attempt. Significantly, American cryptanalysts had provided crucial order of battle and operational communications intelligence to the Allied commanders in the South Pacific.

===AF===

==="AF"===

In addition to this advance toward Port Moresby, evidence that Japan was intent on expanding east of the Marshall Islands appeared in COMINT in early 1942. Land-based air units and equipment began appearing in message traffic to and from the Marshall Islands and the Mandates. On March 4, the designator "AF" began appearing in partially decoded messages. Then, on March 5 Japanese seaplanes, refueled from a submarine at French Frigate Shoals to the east of Midway, conducted a small armed reconnaissance mission over Oahu. Finally, on March 13, American cryptanalysts got the break they were looking for.  Acting on a hunch that "AF" was Midway, they asked the radiomen there to send an open-air message indicating something was wrong with their water supply; the Japanese sent a message in turn indicating the "problem" with "AF".  The result was both the breaking of the Japanese Navy's General-Purpose Code (JN 25) and the identification of "AF" as Midway Island.

In addition to this advance toward Port Moresby, evidence that Japan was intent on expanding east of the Marshall Islands appeared in COMINT in early 1942. Land-based air units and equipment began appearing in message traffic to and from the Marshall Islands and the Mandates. On March 4, the designator "AF" began appearing in partially decoded messages. Then, on March 5 Japanese seaplanes, refueled from a submarine at French Frigate Shoals to the east of Midway, conducted a small armed reconnaissance mission over Oahu. Finally, on March 13, American cryptanalysts got the break they were looking for.  Acting on a hunch that "AF" was Midway, they asked the radiomen there to send an open-air message indicating something was wrong with their water supply; the Japanese sent a message in turn indicating the "problem" with "AF".  The result was both the breaking of the Japanese Navy's General-Purpose Code (JN 25) and the identification of "AF" as Midway Island.