The Venona files were a large set of decryptions which various U.S. intelligence agencies, mainly inside the U.S. Army, were able to intercept and decode as Soviet agents inside the the U.S. sent encrypted messages to their supervisors inside America and in Moscow.
Soviet intelligence operations started in the United States as early as 1919, but reached full force in the 1930s. A network of operatives was in constant contact with the Kremlin, sending data to Moscow, and receiving instructions about how to influence U.S. policies.
Although representing a massive amount of evidence, the Venona decrypts were far from the only data about the international communist conspiracy and its activity in America, as historians Stan Evans and Herbert Romerstein write:
Other revelations dating from the 1990s include material from the archives of the Soviet Union and other east bloc nations when for a brief period after the Communists were toppled from power such records were made available to researchers. The most recent such disclosures are the so-called Vassiliev papers, named for a former Soviet intelligence staffer who made voluminous copies of secret records and smuggled them out of Russia when he defected to the West. Similar revelations had been made by previous such defectors, including Oleg Gordievsky, Stanislav Levchenko, and Victor Kravchenko, along with native American defectors such as Whittaker Chambers and Elizabeth Bentley.
Both Whittaker Chambers and Elizabeth Bentley had worked for the Communist Party (CPUSA) in the United States, which was not a political party at all, but rather a terrorist organization, plotting to send secret military information to Moscow, plotting to influence American policymakers to act not in the best interests of U.S. citizens but rather to the advantage of the USSR, and plotting to eventually use even ‘violent’ methods to overthrow the U.S. government.
The CPUSA had explicitly used the word ‘violent’ in its description of the revolution which it hoped to instigate inside the United States.
After the end of the Cold War, and after the fall of the Soviet Union in 1990/1991, the various sources of data revealed that the extent of the communist espionage network inside the United States was far larger than anyone had previously imagined.