On the question of the Japanese and coded messages, the Japanese like the Germans suffered from a problem. Where the Germans believed that their codes were so sophisticated that they were unbreakable, the Japanese believed that their language was so different and complex that no European would be able to understand it, and thus their codes were unbreakable. This form of national arrogance was as with the Germans, very much their downfall, and by the end of the war vitally all of their codes had been broken. This was helped by a major advantage that the Allies had over the Axis, especially the Anglo Americans, they were prepared to cooperate. It should be noted that Anglo American cooperation across the boards was extensive during WWII, from British scientists working alongside their American and European refugee compatriots on the Atomic bomb project, through numerous technical studies, and a general sharing of intelligence information. With regard to signals intelligence, there were shared signals intercepted sites, and a devision of tasks. The British took the lead on German Enigma. while the Americans concentrated on the Japanese Purple, and the various manual codes were similarly divided, between British and American teams.
The Japanese Army has a number of problems, first it was as I have said before, generally inward looking in comparison to the Japanese Navy. And it placed little internal enfaces on its intelligence services. Being an intelligence officer was in comparison to being a combat officer somewhat dishonourable, nor culturally were they well regarded, as in Japan to disagree with ones superior wasn’t exactly the done thing. In addition up until now the Japanese Army had very little use for an extensive, well trained and equipped signals intercept service, unlike the Navy. They have been fighting the Chinese, who themselves had very little use of Radio/Wireless, and so there was no need to establish a broad interception capability. Whereas the Germans who expected to use Radio/Wireless extensively and expected their opponents to do likewise, did pre war establish such a service. Nor do the Japanese have the large pool of young radio hams, who can speak English, whereas the British and Americans do have such an extensive pool of young men proficient in basic radio/wireless communications. They just need to train them in basic Japanese language skills, or in the case of the Americans use their own Japanese-Americans.
The ability of the Japanese Army to intercept British radio and wireless signals is in comparison to the ability of the Germans to do the same is basic. While their capabilities to break the British codes, is equally basic, given that they haven’t been trying to do so for years, as is the case with the Germans. In addition while the can to an extent intercept voice radio, they have the problem that even their best linguists are going to struggle with some regional accents, and they will have few who understand the various Indian languages. Add to this the British, just like the Americans are not reliant on wireless for their high level communications, having an extensive landline and cable network. High level messages from London to Singapore can be sent over the imperial cable network, and from Singapore to the front lines, on the existing landline system in Malaya. While the Japanese will gain some tactical information from signals interception, they are not going to gain very much strategic information from signals interception. Note towards the end of the war IOTL, the British sent midget submarines to cut the undersea cables between various locations in the Far East, to force the Japanese to switch to wireless communication. Which the British were able to intercept and decipher, I don’t see the Japanese being able to mount such an operation, or having the ability to decipher the British codes.
RR.