Starting as early as 1919, such efforts in the United States reached high levels in the 1930s, and were significantly active for several decades thereafter.
The Soviet espionage network inside the United States was shockingly expansive, and astonishingly effective. A single example suffices: Alger Hiss was a communist operative, and was also a direct advisor to President Roosevelt.
Hiss influenced Roosevelt’s policy-making activities. Some of FDR’s decisions toward the end of his presidency were not in the best interests of the United States, nor in the best interests of its allies, but were advantageous to the USSR.
Hiss’s infiltration becomes all the more surprising when one understands that he was not acting alone, but was part of a large network, and part of a chain of command that led ultimately to Moscow and to Stalin.
How do we know about these Soviet agents? Much of the evidence was not available until the collapse of the Soviet Union around 1990/1991, as historians Stan Evans and Herbert Romerstein write:
Among the information sources now available on such matters, those most often cited are the Venona decrypts compiled by the U.S. Army Signal Corps in the 1940s. Venona was the codename given to encrypted messages exchanged between the Red intelligence bosses in Moscow and their agents in this country. The Army codebreakers intercepted thousands of these missives and by a painstaking process were able to decipher a substantial number. This information, reflecting the extent of the Soviets’ activities in the United States and the identities of many of their contacts, was shared by the Army with the FBI to counter and eventually help break various of the pro-Red networks. These decrypts weren’t made public until 1995, half a century after they were first recorded.
To be sure, some data was available even before the end of the Cold War. Much of that was uncovered and publicized by Congress.
In addition to the Venona intercepts, there were files from the KGB, from the East German Stasi, and from the private records of former Soviet agents who defected to the western nations.
In the United States, some records from the FBI and other agencies were declassified and made public after the end of the Cold War.
Although not all the data is now available, and some of it may be lost forever, there is no doubt that a wide-ranging and dangerous Soviet espionage network existed inside the United States prior to 1990.