In intelligence, the protection of sources and methods is vitally important to continued success. There are few pieces of evidence more convincing than an target’s own encrypted communication, but making it plain that it has been intercepted and decoded is likely to drive the target to tighten security and change up their systems. As such, there is always a balance to be struck between providing authoratative information in the present and retaining the capacity to do so in the future. For example, when Neville Chamberlain read out decrypted Russian telegrams in Parliament in 1927, it led to them switching up their cipher systems and making broader use of one time pads.
All this has consequences for the writing and understanding of history. Roughly, historiography refers to the history and methodology of history. Of particular importance is the history of the lessons drawn from historical events. For instance, the lessons drawn from the two world wars. Very frequently, politicians, historians, and members of the general public draw conclusions without the benefit of access to classified materials, such as intercepted and decrypted military and diplomatic communication.
An example is the Dieppe raid of 1942. In Richard Aldrich’s GCHQ: The Uncensored Story of Britain’s Most Secret Intelligence Agency, the author describes how poor cipher security on the part of the British meant that the Germans had five days warning before the supposed surprise attack. I don’t know when that information became publicly available, but it is a fair bet that it was not until well after many of those involved in the raid had made their private judgments about why it failed.
Arguably, all this is an important reason for continuing to study historical events that are fairly long-past. It might seem questionable what utility there is in studying the Russo-Japanese War in 2010, but one good answer might be how the decreased political sensitivity means that formerly closely-guarded documents are now accessible to scholars. We will probably be waiting many decades before some of the most important documents relating to contemporary international events become open to scrutiny.