IANAL, so the below opinion represents a non-legal reading of the various treaties, obligations and rulings. A judge may well reach a different conclusion. In fact, were Judge Pickles involved (different country so he can't and he retired anyway), any judgement might be possible. The guy was living proof of the razor-edge between genius and utter insanity. However, I feel that even if my reading is legally incorrect, the cited texts should still be taken into consideration.
The Supreme Court has long decided that the Declaration of Independence is just so much scrap paper with no legal backing whatsoever. The argument needs to be stronger.
Now, under US law, all International Treaties that the US has signed up to have the weight of US law. Maybe that will offer some possibilities.
Article 29 of the Second Hague Convention: An individual can only be considered a spy if, acting clandestinely, or on false pretences, he obtains, or seeks to obtain information in the zone of operations of a belligerent, with the intention of communicating it to the hostile party.
Well, there's no claim that he used false pretenses to access the material or that he did so clandestinely. Nor is there any claim that he communicated it to the hostile party.
Article 31 states: A spy who, after rejoining the army to which he belongs, is subsequently captured by the enemy, is treated as a prisoner of war, and incurs no responsibility for his previous acts of espionage.
So if he, after giving the information to Wikileaks, acted correctly under the commanding officer and committed no offence at the time of his arrest would not qualify as a spy as he had "rejoined the army to which he belongs".
Nurenberg Principle II states, "The fact that internal law does not impose a penalty for an act which constitutes a crime under international law does not relieve the person who committed the act from responsibility under international law."
Nurenberg Principle IV states: "The fact that a person acted pursuant to order of his Government or of a superior does not relieve him from responsibility under international law, provided a moral choice was in fact possible to him".
Taken together, this would mean that if Manning's silence would be a crime under international law, then it would be a criminal act even if it was (a) legal in the US and (b) ordered by his superiors. Thus, we now have to establish if his silence was a criminal act.
Principle IV also states:
(a) Crimes against peace:
(i) Planning, preparation, initiation or waging of a war of aggression or a war in violation of international treaties, agreements or assurances;
(ii) Participation in a common plan or conspiracy for the accomplishment of any of the acts mentioned under (i).
Under (ii), silence would be partitipation in a common plan or conspiracy, provided the acts he was aware of were indeed illegal.
Article 5 of the Amelioration of the Condition of the Wounded on the Field of Battle (Red Cross Convention) states: Inhabitants of the country who may bring help to the wounded shall be respected, and shall remain free. The generals of the belligerent Powers shall make it their care to inform the inhabitants of the appeal addressed to their humanity, and of the neutrality which will be the consequence of it.
Thus, bombing civilians rendering aid, regardless of who they are aiding, is an illegal act. Which would make Manning's silence an illegal act under Principle IV above.
So, from this we can reasonably conclude that Manning (a) is not a spy or guilty of espionage (regardless of any US law to the contrary, since international law supercedes it), and (b) would have been guilty of a war crime had he not released the information.
This does NOT make him innocent of any crime. It merely makes him innocent of the crime that is popularly attached to him. There may well be legal grounds for disciplining him for his method of non-silence, but legally he was obliged under international law to be non-silent.