He cares about sound bites and making his base think he can do whatever he wants.

We can have headlines every day about how and why using military for police duties is illegal, he will just say fake news and keep on going.
And he really knows the law, by now, but his base does not know or care, his words are golden to them, especially the agitators.

I know for instance that the 101 Airborne was used to enforce the order integrating Ole Miss.

I don't know what is required though.

https://www.nytimes.com/2002/09/28/opinion/forgotten-soldiers-of-the-integration-fight.html

The military had to defeat a racist insurrection, block by block. Snipers opened fire on army convoys.
The 82nd Airborne was also mobilized.

The military wished to suppress this story about ..."actions involving conflict between US Army units and other Americans".
The Cuban missile crisis a few weeks later helped do so.

They made a really big deal out of it the year I started, because it was the sesquicentennial, and rah rah, 150 years of Ole Miss! Go Rebs!

Shockingly, none of that information was included (that's sarcasm, for the sarcasm-impaired). They glossed right over the reaction to James Meredith's arrival, making it sound as if it was a small group of people who were discontent with integration. Not violently unhinged, but simply discontent.

So even long after the fact, the university wasn't interested in telling the truth, only the convenient fiction.

What could possibly go wrong with trigger happy armed racists waiting at the Mexican border to put a notch on their guns?

"The Posse Comitatus law forbids the US military from enforcing domestic laws, unless there's no other choice. "

If I were in the military, I wouldn't want to be put in the position of having to fire on unarmed, hungry, desperate people seeking asylum, which they have every right to do.