Look up polish involvment in allied intelligence (most notably the breaking of Enigma code).
That is a gross over-simplification. The version cracked by Polish intelligence before the war "really" started was not the same version used during the war. Bletchley Park also came up with other, far more efficient, ways to crack Enigma traffic than the initial Polish approach. And Polish troops were not involved in the (crucial) capture of encryption tables and the new encryption wheels for the four-rotor Enigma (nor was the US Navy, despite the falsified history portrayed in
U-517..., see
http://en.wikipedia.org/wiki/U-559 for details).
However, it is undoubtedly true that the early Polish work gave Bletchley Park a huge advantage. I would actually consider the cracking of the Enigma a great example of the teamwork between various countries on the Allied side, the "no single party did (or could have done) this by itself nearly as well".