The sticky part comes in when the venue for determining the legality of the order often then becomes one's own court-martial and resulting appeals. I'm not sure how much case law there is on the subject.
The law is arguably not what's written, but what actually happens, and analyzed that way our laws about war crime are complicated.
Like, if it's not plainly an awful decision on its face (and god, it so very is) then the fact that just about every plausible application of it looks something like that, as far as who can and cannot be held responsible for their actions, should demonstrate that the decision is, in the highest ideal of what it means to be American, deeply un-American.
The stumbling block is if the President invokes the Insurrection Act. Which is a bit of a gray area.