Reading the press release, it's clear that Libby met the requirements to be charged for violations of the Espionage Act: (1) possession of (2) information (3) relating to the national defense (4) which the person possessing it has reason to believe could be used to damage the United States or aid a foreign nation and (5) wilful communication of that information to (6) a person not entitled to receive it.
Why didn't he get charged with it?
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Under the Espionage Act, the person doing the communicating need not actually believe that revelation could be damaging; he needs only "reason to believe." Classification is generally reason to believe, and a security-clearance holder is responsible for knowing what information is classified.
Nor is it necessary that the discloser intend public distribution; if anyone told a reporter -- which they did -- and that reporter didn't have a security clearance -- which they didn't -- the crime would have been complete.
And to be a crime the disclosure need not be intended to damage the national security; it is only the act of communication itself that must be wilful.
It's also a crime to "cause" such information to be communicated, for example by asking someone else to do so.
If you buy into the bit about Fitz being a competent, aggressive prosecutor, it seems odd that he didn't charge Libby for this violation. The only thing that makes sense is that he's trying to flip Libby, by threatening him with the Big One. Perjury and Obstruction can be plea-bargained down, if Scooter cuts a deal. It looks bad if you cut a deal with someone who violated our secrets laws.
This means Fitz is after someone else. Someone bigger. It could be Rove, who hasn't been indicted because they're trying to get Scooter to flip, or because they have flipped/are trying to flip Rove to get at someone else (most likely the VP).