If he trained them in China and has left the US military it would likely come down to breaching a commitment made when he was a member of the US military. Unless of course you want to allow all countries to try anyone from any country committing any act anywhere in the world under their own laws. Or do you reserve this for the USA alone, western countries only, or who do you limit this to, on what basis and who decides?
Assange is not a US citizen and did not commit any crime in the USA. Where is their jurisdiction? Plus, the USA wanted to try Assange under US law but not offer the protections US law has under the constitution - which is a contradiction.
In any case, he exposed war crimes and told us what our governments were doing and saying on our behalf - I reckon we have a right to this sort of info.
DS