Prior to Hitlers invasion of the USSR, America was content to stay out of the war and the Communists in the US were under orders to maintain that attitude.
Afterwards the policy shifted completely, and Soviet agents went to work in both Tokyo and Washington to create a war between the US and Japan to prevent Japan from supporting their ally Germany with an invasion of Siberia.
FDR immediately shifted his policy toward Japan from appeasement to confrontation, directly resulting in Pearl Harbor.
The highest ranking Soviet spy was the Assistant Secretary of the Treasury, who was highly regarded and had the ear of FDR. The declassification of Project Venona reveals there were at least 400 Soviet spies in high ranking positions in the US government and that they could change policy using misinformation.