I'd put both Snowden and Manning on that list.
Both did what they did to harm the US intelligence and the nation at large, if that isn't treason i don't know what the fuck is.
Look, for some reason people think i'm supporting Snowden because i think it's fucking disgusting that one NATO member spies on an ally who when they called came to help and to physically plant bugs in an office during trade negotiations, if this was domestic espionage the guilty would serve LONG sentences, 30-35 years in the US isn't uncommon for that type of espionage. (rapists get 5 days - life, depending on how drunk the judge is).
Hannsen gave up names of double agents to the KGB, essentially signing their death warrants. He left over 20 dead drops of information for the KGB. In addition to the Wikipedia link provided above, here is the
http://www.fbi.gov/about-us/history/famous-cases/robert-hanssen.
I think your suggestion to "add Snowden to that list" is absurd.
The biggest factor in this is his disclosure of a program that many US citizens equate to a violation of their constitutional rights, something that many of us consider to be virtually holy, and what allowed this country to become great. Many also consider the rise to greatness and any erosion of that greatness to directly parallel how well the Constitution has been adhered too.
I don't think Snowdens actions fit the spirit of espionage, but while I disagree with that, I can somewhat understand why someone outside the US would think otherwise.
Given Obamas propensity to try people under the Espionage act (3 the entire time of its existence vs. 7 for Obama alone!), how was Snowden to disclose this program to the American people? Did he provide technical details to other countries that would allow them to infiltrate or copy the program? I don't know honestly, and that might change things but then again simply duplicating a data stream and retaining it does not cut it as far as an accusation of espionage.
Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort.
That is the definition of treason in the US. From what I know so far, I don't think Snowden fits that definition. I say "so far" because not all of the documents he provided to the media have not been released.
Is Obama guilty of treason for his administrations aid in the development of the Osama raid movie? Is the SEAL who wrote "No Easy Day" guilty?