The breaking of the Japanese code was done without computers or calculators.
The broken code allowed the US to know what was being set up at Midway, and to launch their own attacks. It lead to the first loss for the Japanese navy in over 300 years.
A year or so later, another message was intercepted about the travel plans of Yamamoto, which allowed the U.S. to figure out where he would be and send planes there to shoot his plane down.
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