Alexander Vassiliev's notebooks with 1,115 pages of handwritten transcriptions, excerpts, and summaries from Soviet Committee on State Security (KGB) archival files provide the most detailed documentation available of Soviet espionage in the United States in the 1930s and 1940s. This article discusses the provenance of the notebooks and how they fit with previously available Federal Bureau of Investigation (FBI) files, KGB cables decrypted by the Venona project, Communist International records, court proceedings, and congressional investigations. As an example of the richness of the material, the essay reviews the notebooks' documentation of Soviet spy William Weisband's success in alerting the Soviet Union to the U.S. decryption project that tracked Soviet military logistic communications, allowing the USSR to implement a more secure encryption system and blinding the United States to preparations for the invasion of South Korea in 1950.

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