What did Donald Trump do?

He threatened to give illegal orders to the military.

Riffing on Twitter about immigration this morning, Trump said that if Mexico could not stop a caravan of Central American migrants, he would "call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!"

There are a few problems with this plan--most of them having to do with the fact that the U.S. military obeys U.S. laws. One of them, the Posse Comitatus Act, prohibits using the military as a police force. 

That's why Trump was forced to rely on voluntary donations of National Guard troops from various states for his last attempt at militarizing the border. That didn't go well: National Guard troops sent to the border ended up doing jobs like shoveling manure, and many of the states who sent Guard troops pulled them back when Trump's family separation policy was revealed.

Why does this matter?

• It's bad for a president to say he'll give the military unlawful orders.

• This—
  
  
  
  —is not a threat that requires the attention of the world's most powerful military.