In 2016 there is no doubt that there was in the beginning of the Cold War many hundred Communist and pro-Soviet operating in the West. They managed to wield great leverage on U.S. policy. In some cases they even made or guided key decisions. Classic cases were Soviet agents Harry Dexter White who disinformed Henry Morgenthay and Alger Hiss who disinformed Edward R. Stettinius Jr. 100 of millions of people were added to the Communist sphere of the world. Then China fell to Communism a few years later. Later followed other Marxist conquests in Indochina. Cuba and Nicaragua were added and in Africa countries like Angola. It was not through conventional armies marching across borders, but by actions of subversives inside the nations targeted.
An interesting case in point was the Yalta Conference in 1944,that would doom East European countries to decades of Communist tyranny. For some reason President Franklin Roosevelt brought Soviet agent Alger Hiss along. In most academic studies Alger Hiss role at Yalta id downplayed. In reality he was an outspoken participant in a large number of topics. Stettinius noted in his diaries:
At this point Mr. Hiss brought up the question of China and stressed the importance which the United States attached to U.S., British-Soviet encouragement and support for an agreement between the Comintern and the Chinese Congress in order to further the war effort and prevent possible civil strife.
In the official record of Yalta there is no mention of this.
What influenced Great Britain during World War 2 to choose Tito as partner instead of anticommunist General Draza Mihailovich. Aiding the Tito case in the Balkans was James Klugmann at the British intelligence station in Cairo. Klugmann had been a member of the Cambridge circle of the 1930s with future Soviet spies like Burgess, Maclean, Philby and Blunt. Records exist of Klugmann explaining to an official of the British Communist Party that he (Klugmann) had in Cairo acted to switch British support from Mihailovich to Tito. He was among other things able to control the selection and destination of agents bound for Yugoslavia.
These are only two examples from a book published a couple of years ago. The authors of the book called for more information tracked down in the future. Much of the information of Soviet subversion in America is hard to come by. In the dominating history research there has been a fixation on cases in which subjects were caught passing documents to Soviet handlers. This means that numerous agents, contacts, and episodes with impact on policy matters have been excluded.
Source: Stalin’s Secret Agents: The Subversion of Roosevelt’s Government by M. Stanton Evans and Herbert Romerstein (2012)
Comment: There is much more that deserves more research in the future. Of special interest would be further research on how Soviet influence agents caused the United States to stop supporting the government of the Republic of China. The result was that China in 1949 was conquered by Mao’s Soviet supported military forces.