Joining the armed forces voluntarily means you voluntarily entered UCMJ jursidiction. Espionage is illegal under UCMJ. That's the actual, formal law. It's not a failure of due process.
Manning's torture after his arrest can hardly be considered any part of due process. Obama's refusal to intercede in that situation was what finally soured me on him for good.
Exactly. I would not have objected to Manning receiving multiple life sentences for his violation of the law, if due process had been respected. But it wasn't.
In his defense of Snowden's decision to flee the country, Daniel Ellsberg elaborates more on what's different between now and when he made the decision not to flee in 1971.