One other lesson stands out. The American Intelligence system succeeded in penetrating the enemy’s most closely guarded secrets well in advance of events. Thus Admiral Nimitz, albeit the weaker, was twice able to concentrate all the forces he had in sufficient strength at the right time and place. When the hour struck this proved decisive. The importance of secrecy and the consequence of leakage of information are here proclaimed.
—Winston Churchill. Memoirs of the Second World War, 1950.
Both the Germans and the Japanese had their communications decrypted during World War II. This destroyed Germany’s ability to blockade Britain and to supply its troops in North Africa, and it caused serious problems on the Eastern Front. For Japan the effect was arguably even more significant, namely the defeat at Midway and the severing of its supply lines by American submarines.
The lesson to be learned is that wherever a much vaunted system like Enigma is used, it is likely to have been defeated by the time war breaks out. This is because whatever one ingenious human can create, another ingenious human can eventually defeat. The only safety is in constant movement. The German and Japanese failed not because their codes were broken, but because they failed to change their codes. In the European theater, even as the British studied cracked German signals, their own most important naval and convoy codes had been deciphered by the Germans. Both countries were too arrogant to imagine their own encryption protocols could have been compromised.