http://www.history.co.uk/study-topics/history-of-ww2/code-breaking

During World War II, 

Germany believed that its 

secret codes 

for radio messages were indecipherable to the Allies. 

However, 

the meticulous work of code breakers based at Britain’s Bletchley Park cracked the secrets of German wartime communication, 

and played a crucial role in the final defeat of Germany.

https://www.youtube.com/watch?v=-ITPAbYScIw

The Enigma machines 

were a series of electro-mechanical rotor cipher 

machines 

developed and used in the early- to mid-twentieth century 

to protect 

commercial, diplomatic and military communication. Enigma was invented by the German engineer Arthur Scherbius at the end of World War I.^[1] 

Early models were used commercially from the early 1920s, and adopted by military and government services of several countries, most notably Nazi Germany before and during World War II.^[2] Several different Enigma models were produced, but theGerman military models are the most commonly recognised.

Bletchley Park 

was the central site for Britain's 

codebreakers during World War Two. 

Run by the Government Code and Cypher School (GC&CS), 

it regularly 

penetrated the 

secret communications of theAxis Powers –

 most importantly the German Enigma and Lorenz ciphers. 

The official historian of World War II British Intelligence has written that the "Ultra" intelligence produced at Bletchley shortened the war by two to four years, 

and that without it the outcome of the war would have been uncertain.^[1]