What did Donald Trump do today?

He got confused about how the federal budget works.

Today, Trump retweeted his own tweet from Sunday, in which he declared that his infamous border wall should now be built by the United States military, which he said was now "rich." This came as other administration officials anonymously confirmed that Trump was indeed seriously considering funding border wall construction through the military.

Unfortunately for Trump's plan, federal spending does not work that way. As with every other appropriation for every other part of the government, the Defense Department budget is made up of funds targeted for specific purposes: salaries for military personnel, the purchase of equipment and weapons, development of new technologies, and so forth. Presidents cannot simply ignore or rewrite budgets.

Of course, even if Trump could spend money on whatever he wanted, it would still be American taxpayer dollars at issue--not Mexico, as Trump promised hundreds of times on the campaign trail. Asked about this today, Sarah Huckabee Sanders would only say that Trump "still has plans to look at" exploring ways to send Mexico a bill at some unspecified point in the future. (Behind the scenes, Trump has been reduced to begging Mexican leaders not to publicly contradict him when he talks about Mexico paying.)

Who cares?

• A president who don't know that he's not allowed to spend taxpayer money however he wants is incompetent.
  
• A president who knows this but doesn't care is an authoritarian (and incompetent).
  
• Voters who heard candidate Trump promise them over and over again that Mexico would pay for the wall may have believed that he meant for Mexico to pay for the wall.