America had the VENONA information in the late 1940s--it was based on codebreaking efforts.
Its existence was only disclosed in the 1980s, and the actual decrypts were released only after the collapse of the USSR.
VENONA, due to the nature of the cryptographic break, was only going to be of very limited use--we shouldn't have worried about letting it go public in the 1940s.
VENONA and the Rosenbergs. Too long for me to read now...but for those who are interested forming their own opinions....
Sobell on "Venona and the Rosenbergs"
I'm never going to get to the bottom of this. It's too difficult and too much that's written about it is ideological propaganda.
What is clear is that we were spying on the Russians at least as much as they were spying on us. Why not? The WWII alliance was a marriage of convenience, that's all. Everyone knew the pre-war antagonism would resume as soon as Hitler was gone. Everyone except the idiots. Remind you of the Israeli-Palestinian dispute perhaps?