The Allies cracked both the German and Japanese codes. This was instrumental in defeating Germany and Japan. The intelligence was particularly important early in the war when the information turned critical battles. The Battle of Midway may have been lost without the information provided by deciphered Japanese messages. Most of Rommel’s supplies in North Africa never reached him because the British knew exactly how and when they were coming. they attacked accordingly. If Rommel had been properly resupplied and reinforced, the German probably would have seized the Suez canal. Neither the Germans nor the Japanese became aware that their codes were broken.
Exactly. There were never enough search planes and subs to watch the expanse of the Mediterranean day & night. But if you cracked the sailing schedule you could route your searches to the appropriate areas. And just to ‘throw them off’ you allow yourself to be seen searching other areas as well.