One of the most important gaps in the historiography of World War II that remained in place until after 1974 was focussed on the covert activities of all the states involved. Even with the slow release of Allied intercepted and decrypted signals and records of interrogations of surrendered personnel, the documentary evidence about the secret channels of communication among the Axis Powers tended to be quite thin. The main reason for this lay in the destruction of the papers of the various secret services before the ending of hostilities and in some cases the deliberate avoidance of written compilations. One major example of this is the fact that it is still not fully known how much Hitler depended on the various sources of intercepted telephone calls and wireless signals, the use of bugs, and the burglary of enemy or neutral premises to photograph code and cipher books. As far back as 1923, Hitler himself had been the victim of other agencies who were primed about the planned Nazi coup in Munich and the activities of Weimar agencies soon became available to the Nazi hierarchy in the course of their penetration of government and the military. Parallel moves were also in progress in contemporary Italy and Japan and a pooling of their resources was of key importance to the evolution of the Axis alliance by 1941.
There are a number of fuller accounts of some of these intercept activities, but more often than not there are only snippets which refer directly or indirectly to developments. Wenneker’s War Diary, for example, provides evidence of Japanese interception and decryption of some US State Department traffic in 1941 that was of crucial significance for the preparation of the attack on Pearl Harbor, but also signs of German and Italian assistance to Japan in this field. Evidence has also been found of the role of the Italian intercept agencies of relevance to events in the Indian and Pacific Oceans, including the fact that Italian teams burgled the offices of the Governor-General of Hong Kong in 1938/39, as well as the offices of the Japanese Navy at Shanghai in the summer of 1941.
Japanese officers were credited with assisting the Gestapo to engage in subversive activities against the Soviet Union in the 1930s and to have played a role in bringing about the purges of the Red Army. However, no clear proof of the scale of these activities appears to have survived in archives captured, but there are occasional glimpses into the schemes pursued by the Abwehr from September 1941 against Russia and it is also clear that there were agreements to share information about Soviet wireless codes throughout the war, which were supplemented by the collaboration between Japanese Army officers and Finnish, Estonian, Hungarian and Spanish agencies.