https://www.law.cornell.edu/uscode/text/10/802
the military justice code says quite clearly who can be subject to military justice. Almost all civilians and their crimes are ruled out.
However, McCain was retired military and thus this could apply to him
(4)Retired members of a regular component of the armed forces who are entitled to pay.
It's still illegal for a secret trial and execution though.
This is massively misunderstood by people.
https://www.law.cornell.edu/uscode/text/10/802 the military justice code says quite clearly who can be subject to military justice. Almost all civilians and their crimes are ruled out.
However, McCain was retired military and thus this could apply to him
It's still illegal for a secret trial and execution though.