Was he actually a combatant? I am not aware of him actually picking up a weapon and stepping onto a battlefield. If anything, I think he'd be more akin to a "Tokyo Rose" who exhorted others to treasonous acts. Or if you like, more like a Charles Manson, who didn't actually commit any of the murders for which he received a death sentence. But he was tried in front of a jury. I believe that Tokyo Rose was condemned to death after WWII, however there has been no declaration of war by congress.
It's complicated.
Mark
Tokyo Rose was convicted of treason in 1949 and served a 6 year sentence. She was pardoned by President Ford in 1977 after it was revealed that the most damaging testimony against her was coerced. She died in 2006 at the age of 90.