At long last, after signing a budget that includes $1.375 billion for 55 miles of fence construction—a quarter of the $5.7 billion demand over which he shut down the government for 35 days, and about 5 percent of the $25 billion Democrats offered last year in exchange for the DACA fix he wouldn't approve—noted negotiating expert Donald Trump declared a national border wall emergency on Friday morning. The administration, he announced, will soon direct some $8 billion towards the construction of his most famous campaign promise.

"It's a great thing to do, because we have an invasion of drugs, an invasion of gangs, an invasion of people," he told reporters, referring to a place he visited this week at which there was no evidence whatsoever of these dire-sounding conditions. "It's unacceptable." For those of you wondering whether the latest unhinged Rose Garden press conference means you should go to Costco right now and stock up on bottled water and nonperishable foods, we have assembled this helpful explainer.

What is a national emergency?

In this context, it refers to a power created by the aptly-named National Emergencies Act, a federal law passed in 1976. A key provision of this statutory scheme allows the president, after declaring a state of emergency, to take money Congress has appropriated to the Department of Defense for "military construction projects" and use it to finance other "military construction projects" that purportedly address the purported exigent circumstances. The president may not go beyond the congressionally-established funding levels, however; Trump can shuffle funds around within this pot, but not take funds from other pots for this purpose.

Colloquially speaking, it is also shorthand for everything that has occurred in the White House since January 20, 2017.

That sounds serious. How rare are national emergencies?

Not very. More than two dozen are already in place, most of which relate to the imposition of economic sanctions; one, which provides the basis for enforcing such sanctions against Iran, has been renewed every year by presidents of both parties since its genesis during the Carter administration. Trump reiterated this fact several times during his address, in an effort to show that no substantive difference exists between this declaration and the many that preceded it.

Does such a difference exist?

Yes. To spell it out: Trump is invoking emergency powers here not because of any unforeseen, dangerous circumstances that suddenly arose and merit an immediate response; he is doing so because Congress would not give him a thing he wants.

It is a transparent attempt to circumvent the constitutionally-enshrined appropriations process, ignore the will of voters (as expressed in both wall-specific polling numbers and also through their choices of representatives in Washington), and stuff the separation-of-power principles upon which American democracy rests into the trash.

Wow. Is there any check on this behavior? Is he going to get away with this?

Yes, and maybe. Once the president declares a state of emergency, the NEA allows the legislature an opportunity to end it by passing a joint resolution. As is the case with a bill, a joint resolution must be approved by both chambers in order to make it to the president's desk. If he vetoes it, Congress can override that veto with a pair of two-thirds majorities. If he signs it—or if a veto override attempt fails—the NEA effectively requires Congress to conduct a periodic review of the situation by reconsidering a new emergency-terminating joint resolution every six months.

It is a safe bet that in the House, Nancy Pelosi and company will do their half of the work immediately. Trump's task, then, is to get at least 50 senators to agree with him that, yes, a national emergency at the southern border—again, a place at which he spoke days ago without incident—in fact exists.