The Enigma machine was first adopted by the German military in the Wehmar Republic in the late 1920s.The Germans thought the Enigma machine was safe because of the mathematics of it (millions of combinations).
When tension between Poland and Nazi Germany was high, an Enigma was accidentally shipped to a Polish warehouse before the start of WWII. The Germans immediately realized they had made the mistake and were anxious to get it back. The Polish Intelligence Service realized it was probably important because of their desperation to get the crate back, they examined the Enigma and realized they had a German coding machine and decided it would be more valuable if they did not let the Germans know that they knew about their code machine. They took photos, made drawings, and reassembled the crate. Germans had no idea that the Poles had done this. The British had in their possession an Enigma machine, so they knew the process and how the machine worked. The code could be constantly adjusted, meaning that it would have to be decoded every time. For the next few years experts work to understand the code and by 1942 they are fully able to read all.

German U-Boats would receive they orders at sea through Enigma machines and the “MI5” in Bletchley Park worked out all of the codes and discovered algorithms. MI5 agreed to share information with the US but not with the Soviet Union. Stalin is never told about the interceptions, but Soviets had their own sources of interception (spies). The US had representatives at Bletchley park. The allies could not be too obvious that they were able to decipher German messages, but they could track down double agents and determine how many men Germany was sending to each battle. Rather than arresting double agents, they made sure those people didn’t have access to the real information and fed them false information.