There were Communists--actual Soviet agents--in the State Department. Not that there's anything wrong with that . . .And the truth is that Truman, and certainly FDR, actually thought that. It wasn't that they were oblivious to evidence against Soviet agents in their government nearly as much as it was that they did not view that possibility with alarm.
The reality is that FDR liked the Soviet Union better than he liked Britain, and started seriously wanting to get into the war against Hitler when Hitler invaded the Soviet Union. Read The New Dealers' War by self-described Truman Democrat Thomas Flemming; he's upfront about most of the above. Notably missing from his account, in which he states that FDR had the Navy harassing U-boats in the Atlantic "throughout the summer of 1941" is the date of Hitler's attack on Stalin--June 22,1941. Sounds amazingly close to the first day of summer, doesn't it?
The book opens with an account of a brouhaha now otherwise forgotten--the publication in a Chicago paper of the War Department's plan--such as it was--for fighting the Axis. This was somewhat controversial because lots of Americans didn't know anyone who wanted the US to enter WWII. Public sentiment ran 80% against it, so the idea of a government plan to have a million Americans under arms was a bit off-the-wall. There was a protest of that plan by the FDR Administration the following Sunday. December 7, 1941 . . .
What a stroke of luck for FDR! The isolationists--representing the preponderant sentiment in the country--were protesting the idea of fighting WWII just as bombs were falling on Pearl Harbor!
The chief suspect of the leak to the Chicago paper went on to have a fine career in the Army. And as we only learned decades later, FDR had had warning before the leak occurred of what the Japanese mission had been ordered to say at a specific hour on December 7 . . .
Print up the headline, hold it up in the Senate:
FDR He called the ball, the shot, and the pocket. And then went to work shipping war supplies to--Joe Stalin and the Soviet Union.KNEW!