Before the famous Bletchley Park efforts, three Polish mathematicians—Jerzy Różycki, Henryk Zygalski, and Marian Rejewski—had already cracked the German Enigma machine in 1932. They built an electromechanical device called the "bomba" to automate decryption. When Poland was invaded, they fled to France and then Britain, transferring their knowledge to the Allies.
Their work shortened the war by an estimated 2-4 years and saved countless lives. Rejewski once said he felt like he was "fighting with a shadow."