FDR, and I would argue his administration, fully understood the extent to which the Russia could potentially be a threat. Under FDR's administration, the United States began extensively collecting intelligence from the USSR and worked to "limit the expansion of Russian power in 1945 by refusing to share the secret of the atomic bomb." (Dallek). The "threat" of the USSR was largely offset by FDR's relationship with Stalin whom Frank Costigliola has argued to be cautiously optimistic. Here, "Roosevelt hedged his bets on post war cooperation" keeping info of the atomic bomb and post-war economic aide "up his sleeve."

When FDR died and Truman took his place, the relationship between Stalin, Churchill and the United States soured adding to a growing list of post-war grievances the USSR had with the US. It was around this time that the the "threat" of the USSR became very real.

"Roosevelt's Lost Alliances:How Personal Politics Helped Start the Cold War"-Frank Costigliola

"Franklin D. Roosevelt and American Foreign Policy, 1932-1945 Book"-Robert Dallek