Jan Kowalewski
Japanese authors have identified two events that influenced the Japanese army's decision to invite a foreigner to improve their cryptology.
The first was an incident during the Siberian Intervention. The Japanese army came into possession of some Soviet diplomatic correspondence, but their cryptanalysts were unable decipher the messages. Someone suggested asking the Polish military to try cryptanalyzing them. It took the Poles less than a week to break the code and read the messages.
The second event also involved a failure to decipher intercepts. Starting in 1923, the Army began intercepting European and American diplomatic radio communications. Interception was difficult but the task of deciphering intercepted messages proved too much for the Army cryptanalysts.
These two failures convinced the leaders of the Japanese army that they needed some outside help and for geopolitical reasons, they decided to turn to the Polish military. Poland had fought the Soviet Union in 1920 and the Japanese believed the Poles would be receptive to the idea of teaching someone on the Soviet Union's opposite flank how to read Soviet codes.
Read more about this topic: Japanese Cryptology From The 1500s To Meiji