I'm not an expert on relevant US legislative acts, but this is the legal definition in local laws here and I expect that the term of espionage have been defined a few centuries ago and would be mostly matching throughout the world.
A quick look at current US laws (http://www.law.cornell.edu/uscode/18/usc_sec_18_00000793----000-.html) does indicate that there is a penalty for such actions with 'intent or reason to believe ... for the injury of United States or advantage of any foreign nation' - so simply acting to intentionally harm US would be punishable as well, but it's not calling it espionage. And the Manning issue would depend on his intention/reason to believe about harming US vs. helping US nation, which may be clarified by evidence in his earlier communications with Adrian Lamo and others.
Whether Assange is intent on helping the US nation or damaging depends on how you define "the US nation". Assange likes the US constitution but hates the current US government.
If you try to let the government crumble with the goal of regime change to get a regime that honors the US constitution is that damaging the US nation?
Assange wrote in one of the interview that founding Wikileaks was a "forced move". Why is it a forced move? Because otherwise the war was lost. Which war? http://events.ccc.de/congress/2005/fahrplan/events/920.en.h...
Sun Tzu said, "The supreme art of war is to subdue the enemy without fighting." This is also true in rhetoric. The best way to get a belief accepted is to fool people into thinking that they have already accepted it.
(Note, first-year students, that I did not say, "The best way to convince people of a belief". Do not try to convince people! It will not work; and it may start them thinking.)
An excellent way of doing this is to embed your desired conclusion as a presupposition to an enticing argument. If you are debating abortion, and you wish people to believe that human and non-human life are qualitatively different, begin by saying, "We all agree that killing humans is immoral. So when does human life begin?" People will be so eager to jump into the debate about whether a life becomes "human" at conception, the second trimester, or at birth (I myself favor "on moving out of the house"), they won't notice that they agreed to the embedded presupposition that the problem should be phrased as a binary category membership problem, rather than as one of tradeoffs or utility calculations.
Consider the recent furor over whether WikiLeaks leader Julian Assange is a journalist, or can be prosecuted for espionage. I don't know who initially asked this question. The earliest posing of the question that I can find that relates it to the First Amendment is this piece from Fox News on Dec. 8; but Marc Thiessen's column in the Washington Post of Aug. 3 has similar implications. Note that this question presupposes that First Amendment protection applies only to journalists! There is no legal precedent for this that I'm aware of; yet if people spend enough time debating whether Julian Assange is a journalist, they will have unknowingly convinced themselves that ordinary citizens have no First Amendment rights. (We can only hope that this was an artful stroke made from the shadows by some great master of the Dark Arts, and not a mere snowballing of an ignorant question.)