What did Donald Trump do today?

He made an empty threat.

Trump posted this on Twitter today:

No, he may not—and no, he cannot.

As this site noted exactly three months ago, the last time Trump repeated this threat:

Looking only at the economic effect, it's hard to overstate how disastrous this would be. Mexico is the United States' third largest trading partner, and the two countries do about $1.7 billion dollars in trade every day. Any interruption in legal trade would paralyze American industries ranging from automaking to agriculture. As Trump himself has claimed (more or less accurately), there are about a million legal crossings per day, most of them having to do with trade or business.

Also—for whatever it is worth in the Trump administration—he has no legal authority whatsoever to do any such thing.

Trump's claim about Mexico is also a lie. Mexico does devote significant resources to what it calls Programa Frontera Sur, a military operation to intercept migrants at its southern border—for now. Trump's obsession over historically low numbers of illegal border crossings has given Mexico leverage over the United States it didn't have before.

Why does this matter?

• When presidents make empty threats it undermines the presidency and the United States.

• It's wrong to lie.

• Other countries act in their own best interest, not the United States', no matter what a president tweets.