Book Ciphers vs Mechanical Ciphers: The Codes of WW2

In both the fights against the Germans and the Japanese, cracking their codes was key to gaining vital information. However, in each case, the type of code that needed to be broken was different. In Japanese codes, for example, the J-25 code, a message would be written and then translated with a 'book' into the coded message, so these were called 'book ciphers'. These were easier for the Americans to crack by simply using the context of the messages and cross-referencing them with other messages that they had intercepted. When a code was broken, the Japanese would simply change their codes, however, due to the extensive number of words they needed to have codes for, they didn't change the codes enough each time, which allowed the Americans to crack the code with even more ease the next time.
The German system of mechanical ciphers was a little more complex. Using machines like the Enigma machine, a message was encoded with a series of replacements and reflections to different letters. So, instead of just replacing words, it was replacing individual letters and it was very difficult, if almost impossible to decode the messages without either having one of the enigma machines or knowing the exact series of steps that were taken within the machine. After the Americans broke the code and the Germans figured it out, the Germans added an additional wheel into the machine, which multiplies the possibilities of different combinations by 26 because there was another set of changes, so a letter could be any letter of the alphabet again. The British called this new code "Shark" and broke it in December 1942.
While there were many different systems used by the Axis powers to send coded messages, the organization that can be attributed to breaking the codes is Bletchley Park in Britain. It has been said that the work that was done there shortened the war by what could have been an additional 2 or 3 years. At the height of its work, this organization employed 10000 people, which up to three-quarters of which were women. At this point, it was not simply an organization to break codes but to deal with all war intelligence, and utilize the information they found in order to help win the war.

https://slate.com/human-interest/2013/11/u-s-in-world-war-ii-how-the-navy-broke-japanese-codes-before-midway.html
http://practicalcryptography.com/ciphers/mechanical-era/
http://practicalcryptography.com/ciphers/mechanical-era/enigma/
https://www.history.co.uk/history-of-ww2/code-breaking
https://www.historyextra.com/period/second-world-war/bletchley-park-facts-alan-turing/