There are only about 92,566 articles on the Internet about why Donald Trump's "State of National Emergency" is an unconstitutional sham. This one by Bruce Fein is as good as any and it has the advantage of coming from a solid conservative legal authority. The problem with it of course is that none of this law mumbo-jumbo matters to Trump. It never has; not in business, not in government, not in life. If it gets him what he wants, he does it, end of argument. You might as well explain to a lovesick cocker spaniel why he shouldn't hump your leg.
The thing is: I don't think what Trump wants is to build his fakakta wall. I think he just doesn't want to be blamed by his base for not building it and also doesn't want to be seen as inept at delivering on his promises. In his little speech about how it would be challenged in court, he was assuring his supporters that while the wall will probably lose in lower courts, he'd win in the end once it got to the Supreme Court. That's another way of saying, "Don't fault me if it takes a while. It's because of those evil Socialist Democrats."
In the meantime, here's William Saletan shredding the various arguments Trump backers are using to defend his actions.