This isn’t a question of law. The law is entirely clear that federal agents (a police force) have dominion over the border. They shouldn’t have gone through the courts in the first place, they should have simply showed up with a bunch of federal agents and started arresting anyone who stopped them. There’s nothing unclear about what the constitution says happens here. You don’t need a court to say blocking a federal agent from doing their job is illegal.
They don’t need to bring in the military unless armed agents of Texas start resisting, in which case it is insurrection and the military is the right tool for the job. This idea that maybe we should just let Texas play at rebellion for 9 months until the court tells them (again) that federal supremacy is real is the ridiculous.