I agree with Paul's comment regarding using the Army. Well, let me clarify: It should be illegal. But, all Trump has to do is say they are doing drug interdiction. It worked in Waco against US citizens for the Democrats, so it should work against non-US citizens for the GOP. But, it should have been illegal in both cases. Law enforcement should handle these situations. Having said that, the argument for using the Army is stronger against non-citizens who crossed the border illegally (which is what Trump wants to do) than it is for using it against citizens (which is what the Democrats did). See how these precedents by the Dems come back to haunt them? If the Army cannot be used to defend the border, then it has no purpose whatsoever.
Paul disagrees with Trump on tariffs as a policy matter, not a matter of legality. I think libertarians are a bit naive when it comes to free trade. Yes, it should be the ideal, just like widespread immigration should be the ideal. But, there are times, and manners, in which it is not appropriate.