Gov. of TX orders national guard to take control of border crossing, prevent border control from entering area.

The area was city property

TX Gov seized it, now it's state property

At a high level this is constitutional because county/city gov'ts are basically 100% state gov't's b-word, they have no independent sovereignty

At a lower level TX will have some laws to regulate how that works and I don't know them, but something about emergency powers is mentioned, let's assume that checks out

So now:

Migrants entering are getting picked up by TX for criminal trespass on closed state land. TX can't enforce immigration law, but it can enforce trespass.

Border Patrol presumably was there with the permission or at least acquiescence of the mayor (who is against the governor's action).

But now the response to Border Patrol trying to enter is "Where's your frickin' warrant?"

I think it helps that judging by the quote from the union, the actual Border Patrol guys are cool with it

I think it's strictly federal but maybe Texas has grounds to say that the feds aren't doing their job and therefore they have the right to step in.