He should have been tried and executed in the field. This shouldn't have come back to the USA at all.
I agree summary punishment should have happened immediately after the incident.
Exactly.
Treason on the battlefield is an instant death sentence carried out by whoever has enough brass on their shoulder(the amount varies inversely with how big the scumbag is. In this case a E-1 would work).
Uniform Code of Military Justice (UCMJ)
Military Law and Legal Links page (including amendments to UCMJ)
Supreme Court of the United States (Search SCOTUS files)