The Venona Project was begun in 1943 by Colonel Carter Clarke, chief of the U.S. Army's Special Branch, in response to rumors that Stalin was negotiating a separate peace with Hitler. Only a few years earlier, the world had been staggered by the Hitler-Stalin Pact. Unaccountably, Colonel Clarke did not share President Roosevelt's trust in the man FDR called "Uncle Joe." Cloaked in secrecy, Clarke set up a special Army unit to break the Soviet code. Neither President Roosevelt nor President Truman was told about the Venona Project. This was a matter of vital national security: The Democrats could not be trusted.
Historians have hotly debated the degree to which FDR was taken in by Stalin. FDR did manage to talk Stalin into at least making the gesture of preparing to engage the Japanese in a militarily significant way, and this may have sped up Japan's timetable for surrender. So maybe FDR wasn't always easily fooled. Truman also had his doubts about Stalin's integrity. Yet both presidents were lulled into thinking that Stalin's word, or his signature on a document like a treaty, had meaning. It was clear that Stalin would say anything, or sign any document, while planning to do the very opposite.
The Soviets used a code that was, in theory, unbreakable. But by the war's end, the Americans had cracked it. And when the Venona cryptographers read the Soviet cables they discovered something far more sinister than Stalin's war plans: The Roosevelt administration was teeming with paid agents of Moscow. Stalin's handmaidens held strategic positions at the White House, the State Department, the War Department, the Office of Strategic Services, and the Treasury Department.
This discovery provoked a crisis. People had been appointed or hired into positions which gave them access to sensitive information - people whose stated and sworn objective was to destroy the United States. Men and women were working in departments and making decisions about national policy - men and women who were attempting to implement a plan for a communist takeover in America, a takeover which would have eliminated political freedom as we know it.
Given the large volume of communications between Soviet agents in the United States and their headquarters in Moscow, "only a small number of the intercepted Soviet cables have been decoded. But even that much proves" that America was in grave danger. "The U.S. government had a major Communist infestation problem." In hindsight, FDR's and Truman's "gravest error was in underestimating the problem of Communist subversion."