Trump has vaguely threatened to declare a national emergency and order the military to build his fantasy border wall if Congress won't give him the money for, but how would that actually work? How would that be paid for?
According to congressional aides who spoke to the Wall Street Journal, ordering the military to build and a pay for wall would require dipping into funds that were appropriated for completely different purposes.
In other words, Trump would have to take money away from The Troops™
Currently, $13.3 billion in the Pentagon budget may be available, according to a congressional aide, enough to cover the $5 billion that Mr. Trump is seeking for the border wall. That would have to be diverted from projects such as military housing that Congress previously authorized.
Costs for the wall likely would include not only design and construction, but also purchase, potentially through condemnation, of private property along the border.
I don't know if Trump is going to do this, but I would not necessarily be surprised if he tried.
In that event, I expect Senate Majority Leader Mitch McConnell would shield him from congressional action and federal courts may or may not intervene.
If Trump does follow through on his threat to use the military in this way, high level resignations at the Pentagon may be a warning sign.