With recent threads on government "spying" on their citizens, I believe on aspect of our government, which very much flies under the radar and is very much over looked, is Operation Venona.
https://en.wikipedia.org/wiki/Venona_project
This is a great overview, however alot of its actual outcomes were left out. Venona is the codename for millions of Soviet files, located after World War II. It took many years and, in fact, many files still have been decoded.
However many soviet spies were located with Venona and many more implicated. One issue the government had at the time was the fact if they started to round up all the spies Venona uncovered, would have made the Soviet's aware of our code breaking.
So, the US Government came with the "Red Scare." It accused many people (most rightfully so) of being communists. Arrested them, along with the true spies, so as to make the soviet government believe it was just a witch hunt and of course several of their spies were caught up in it.
The sad part of the program was the number of real, true American citizens who were caught up in this mess. MANY Americans were blacklisted and had their career's and lives ruined behind. All done by the government agencies and to protect their valuable intelligence sources.
What is going today, pales in comparison.