On February 1, 1943, the U.S. Army's Signal Intelligence Service (SIS) -- a forerunner of the National Security Agency -- launched a top-secret program, later codenamed VENONA (a word with no special meaning), whose aim was to decipher, and possibly exploit, encrypted Soviet diplomatic communications that had been accumulated by the SIS since 1939. The Venona Project was a collaborative effort between American and British intelligence agencies.

Most of the Soviet intelligence messages which these agencies would ultimately decode had originally been transmitted between 1942 and 1945. Sometime in 1945, SIS analyst and cryptologist Bill Weisband, who doubled as an espionage agent for the Soviet Secret Police, or NKVD, revealed the existence of the Venona program to the USSR.

The Venona decrypting initiative began in 1946 and supplied the Western powers with valuable information on Soviet espionage activity in the early years of the Cold War. The project continued until 1980; during that 34-year period, it was known by at least 13 different code words, "Venona" being the last one used. According to authors John Earl Haynes and Harvey Klehr, the Venona transcripts identify 349 Americans who had a covert relationship with Soviet intelligence. The Venona decryptions were crucial in establishing the espionage activities of such luminaries as Julius Rosenberg, Alger Hiss, and Harry Dexter White.

The Mitrokhin Archive consists of the collected notes that Vasili Mitrokhin, a senior archivist for the Soviet Union's foreign intelligence service and the First Chief Directorate of the KGB, had made over a period of 30 years. Rife with details of Cold War-era Soviet intelligence operations, these notes became public in the aftermath of Mitrokhin's 1992 departure from Russia to the United Kingdom.

Mitrokhin's documents showed, among other things, that more than half of all Soviet weapons systems were based on designs that had been stolen from the United States, often by spies who had infiltrated America's leading defense contractors. The papers further revealed that the KGB had: (a) tapped the telephones of high-ranking American officials, including Secretary of State Henry Kissinger; (b) infiltrated the governments of France and Germany; and (c) planned large-scale sabotage operations against the United States and Canada. According to the FBI, the Mitrokhin Archive was "the most complete and extensive intelligence ever received from any source."