What did Donald Trump do today?

He made promises about military Dreamers that he himself says he doesn't have the authority to keep.

Speaking for the Trump administration, Secretary of Defense James Mattis today announced that the 850 DACA recipients currently serving in the military will be protected from deportation. "They will not be subject to any kind of deportation," Mattis said. "In terms of the DACA situation ... it's clarified they are not in any kind of jeopardy." 

"Dreamers" are extremely sympathetic with Americans, and protecting those serving the United Sates in uniform would be popular. But according to Trump's own supposed legal rationale for rescinding the executive order that created the DACA program in the first place, he cannot make any such promise. "There can be no path to principled immigration reform if the executive branch is able to rewrite or nullify federal laws at will," Trump said at the time. 

Mattis's promise was itself qualified: servicemembers could still be deported if they were subject to a specific deportation order by a judge. Nothing would prevent federal authorities for seeking such an order if a military DACA recipient came to their attention. Under Trump, Immigration and Customs Enforcement has begun targeting immigration activists for deportation.

What's the problem with this?

• Presidents' principles shouldn't change based on what's politically convenient.

• You don't get credit for partially solving a problem of your own making.