Can a non-US citizen be charged with espionage? Sounds like that would be illogical. Julian is not a US citizen. Sorry not a law fag. If this is so freaking obvious, I apologize.
Sure, spies all the time if they wanted to rather than exchange them for our spies who were caught. The Germans caught during WWII and executed, 6 of the eight weren't citizens. Case became precedent for trying 'unlawful combatants' by military tribunal.
Espionage - Yes
Espionage is the act of obtaining secret or confidential information without the permission of the holder of the information.
Treason - not so much
Treason is criminal disloyalty to the state. It is a crime that covers some of the more extreme acts against one’s nation or sovereign.
Yes, crimes committed in or against the United States can be prosecuted against anyone except a diplomat who has immunity. Espionage, i.e., stealing confidential information or gathering information for use by a hostile power, is a crime under the US Code. I haven’t looked it up, so I can’t quote the specifics of what conduct is illegal, but we have prosecuted lots of spies over the decades. Soviet embassy staff couldn’t be prosecuted but spies in the US or elsewhere in the world who illegally obtained information could theoretically be prosecuted if they were caught and extradited. If they were uniformed soldiers acting in combat, that’s a whole different issue, to which the Geneva Convention applies.