The nations involved fielded a plethora of code and cipher systems, many of the latter using rotor machines.
Possibly the most important codebreaking event of the war was the successful decryption by the Allies of the German "Enigma" Cipher.
The first break into Enigma was accomplished by Polish Cipher Bureau around 1932; the techniques and insights used were passed to the French and British Allies just before the outbreak of the war in 1939.
[1][2][3] A similar break into the most secure Japanese diplomatic cipher, designated Purple by the US Army Signals Intelligence Service, started before the US entered the war.
On the other side, German code breaking in World War II achieved some notable successes cracking British naval and other ciphers.