At its heart, the Fuchs story is about knowledge – how it is created, how it is used and misused, and how it is protected from friends and enemies. Among several ironies is that necessary but extreme secretiveness ultimately compromised both British and American security. One explanation for the failure to investigate Fuchs properly was that the officials responsible for security vetting did not know the importance of his role. Thanks to Fuchs, therefore, Joseph Stalin knew considerably more about what was going on at Los Alamos than almost everyone in the American and British governments: compartmentalising that knowledge hindered the Allies but failed to protect the nuclear crown jewels. Similarly, the Americans’ deciphered [Venona] intercepts of Soviet diplomatic communications – which yielded the clues that led to Fuchs – were almost as tightly held, causing numerous difficulties in identifying Fuchs as a spy, persuading him to confess, and ensuring his conviction.

https://www.newstatesman.com/culture/books/2019/08/klaus-fusch-spy-atomic-age