You guys keep creating hypotheticals which are already prohibited by law. So, yeah, something that is already prohibited by law is "obviously illegal."
But a deployment of troops to Chicago or a declaration of war on Venezuela is not already prohibited by law. If you want it to be an "illegal order," it must be against an existing law.
For that matter, "harming civilians" is also not prohibited by law. If civilians were to open fire on military that had been deployed to Chicago, returning fire is not prohibited by law.
I had mentioned before that one possible angle behind this is a "if our legislation doesn't pass" hedge/backup plan.
Elissa Slotkin and Mark Kelly just recently introduced a bill (though, I don't believe it's reached the floor for at vote yet, it still may be in committee) aimed at making the kinds domestic military usages in question explicitly illegal and involving more congressional input/approval.
Right off the bat, it tells us something: It's not already "obviously illegal", if it was, they wouldn't be having to propose a brand new law to make it illegal.
Where I think it's a "hedge" is that with the current makeup of the legislature in both houses, the numbers aren't on their side right now. There's a good chance that'll change after the mid-terms, but that's still a ways off which means they won't have a remote chance of having the numbers to get it through with a veto-proof majority until the new congress starts up in 2027.
However, if they can create enough inner-turmoil within the military ranks that creates confusion and doubt about whether or not to follow orders, that can effectively slow Trump's plans to a crawl until they have the numbers to get actual legislation in place.
This isn't an entirely new concept for congress or former administration appointees -- that concept being, using unelected, like-minded federal employees as a bulwark against a president's agenda as part of the "resistance".
One notable example (and this is from a 2017 WaPo piece, so definitely not conservative bias)
There’s another level of resistance to the new president that is less visible and potentially more troublesome to the administration: a growing wave of opposition from the federal workers charged with implementing any new president’s agenda.
Less than two weeks into Trump’s administration, federal workers are in regular consultation with recently departed Obama-era political appointees about what they can do to push back against the new president’s initiatives. Some federal employees have set up social media accounts to anonymously leak word of changes that Trump appointees are trying to make.
Career staff members in at least five departments said they are staying in close contact with Obama-era administration officials to get advice on how to pushback against Trump initiatives.
Former labor secretary Thomas Perez, who also headed the Justice Department’s civil rights division under Obama, said he has been working to mobilize grass-roots opposition.
...so not novel with regards to the general approach.
However, what is different about this, is that typically they've leveraged like-minded career federal employees and staffers in places like the State Department for this kind of of bulwark "bring it to a screeching halt" tactic. To my knowledge, they haven't done it with the military servicemen before.
The stakes are much different in this instance.
If a state department employee who dislikes Trump opts to feign incompetence to make a project go poorly, call off sick a bunch, intentionally slow-walk a project to bungle an initiative, they're not going to find themselves in front of a judge, they're not going to get fired (because they're protected by powerful unions)
If a servicemen is given the order of "Okay, protestors have blocked road access to an ICE facility and federal agents can't get in or out, and the governor has refused to do anything about it and ordered state police to not intervene, our job is go clear them out of the way", and that servicemen says "Well, two senators and four former members of the state department put out that video saying they thought that was illegal, and that I could even get in trouble for following that order" and then refuses the order, there will be a very different set of consequences that happens.