And if he had been cought out of uniform, as several were that the germans put ashore by U boat, he would have been tried and hung.
But not, you will notice, he would still be given a trial (no deprivation of liberty without due process). Similarly, if he were in uniform, he would still have been given a status hearing (as demanded by the Geneva Conventions) and offered a chance to prove that he wasn't a Wehrmacht agent, but an idiot on his way to a costume party?
If I recall, Casablanca was released in 1942 and featured several characters in full Nazi/Wehrmacht regalia. Would it have been fair, legal, or reasonable for Federal Agents to barge onto the set and arrest Conrad Veidt for the costume he was wearing? If that had happened, should Mr. Veidt be given any opportunity to protest his treatment and to petition the government for release?
Common sense says "yes."
The Geneva Conventions say "yes."
The Constitution of the United States says "yes."
The SCOTUS says "yes."
.... but Bush said, and as far as I can tell, continues to say, "no."