He declared war on the USA by his actions and deeds. That made him a combatant though not necessarily of any country. If you were to go overseas and start attacking American troops what do you think would happen to you? You think the FBI will come for you? And again, he was a 14th. Amendment citizen with no allegiance to the US whatsoever.
But Congress did not declare war. So unless there is a clear and present danger, an imminent threat he was assassinated.
I don’t recall a lot of trials for Americans caught fighting for Hitler. As my granddad said, they chose to eliminate themselves from history and we were happy to oblige.