you're arguing from an ethical viewpoint, which is fine, but it does not apply to this sentence. i know this is hard, but let me explain.
in the civil justice system there are laws. if you break them, you get punished. some of them have special rules around them. take murder. kill someone, go to jail. but if it was self defence, you might walk. easy, right?
in the military justice system, leaking classified docs is a punishable offence. there is no way around this. there is no self defence, etc alternate way. you break the rule, you get court martialed. morals, ethics do not come into play here.
as a soldier, your interpretation of your oath does not apply here. it would when refusing orders like "kill those civilians over there", but only maybe and not in the way you'd like. again, military justice is not following the same framwork of rules and ethics you're used to as a civilian.
the armed forces in itself are not a democracy. they are a special area of society that exist to provide the ability to kill other humans. in order to do so anything civilian quickly goes out the window. the rules exist to enforce the most optimal sytem to maintain a killing force, not the well being of the individual within the organization.
if you don't like that, you should campaign for complete disarmament and the complete disassemblement of the armed forces. there is no half-way, grey area. once you have an army, it exists to kill humans, with all the consequences. the hero whorship present in the US around any soldier seems to imply that the US society has forgotten this (and then is surprised once something like the sentencing of Manning happens).
Nuremberg does not apply at all here. German officers were tried by the Allied Forces, not their own Army. This was the victors cleaning up. The US has not joined the International Criminal Court for similar reasons, to prevent other nations prosecuting its officers and soldiers (for war crimes, etc.). If you want to do so, you'll need to invade the US and be the victor. Note that Saddam was tried and sentenced by an Iraqi court, not an international tribunal even though US soldiers caught him.