The White House says President Donald Trump will declare a national emergency and take other executive action to allow construction of the president’s long-promised southern border wall, after bipartisan congressional negotiations provided less than a quarter of the $5.7 billion Trump wanted to start building more than 200 miles of wall.

There is less than $1.4 billion in the budget deal the Senate and House approved Thursday — enough for just 55 miles of new barriers and fencing.

White House Press Secretary Sarah Sanders said Trump would sign the bill and take “other executive action, including a national emergency.” The declaration would allow Trump to deliver “on his promise to build the wall, protect the border, and secure our great country,” Sanders said. Other executive action could allow Trump to tap other pots of money without the emergency declaration.

Declaring a national emergency at the southern border would prompt a legal fight that would test the powers of all three branches of government. Here’s what we know:

WHY AN EMERGENCY DECLARATION?

The administration has spent months trying to figure out how the president might be able to move forward with the wall — the central promise of his 2016 campaign — if Congress refuses to give him the money.

As early as last March, Trump was publicly floating the idea of using the military for the task. “Building a great Border Wall, with drugs (poison) and enemy combatants pouring into our Country, is all about National Defense. Build WALL through M!” he tweeted then.

But it’s Congress — not the president — that controls the country’s purse strings and must appropriate money he wants to spend.

Among the laws Trump could turn to is Section 2808 of Title 10 of the U.S. Code pertaining to military construction.

According to the statute, if the president declares an emergency “that requires use of the armed forces,” the defense secretary “may undertake military construction projects, and may authorize the Secretaries of the military departments to undertake military construction projects, not otherwise authorized by law that are necessary to support such use of the armed forces.”

Another law, Section 2293 of the code’s Title33, allows the diversion of funds from an Army civil works project to a mission that is “essential to the national defense.”

Congressional aides say there is $21 billion in military construction funds that could potentially be used for a wall in the event Trump declares an emergency.

There is about $10 billion in funds from the current 2019 fiscal year that ends Sept. 30, and another $11 billion from the previous four years that haven’t been obligated or contracted for a project, the aides said. They spoke on condition of anonymity because they weren’t authorized to speak publicly about the funding details.

The Defense Department has declined to provide any details on the amount of money available. The congressional aides said their data came from the Pentagon.

As an example, the aides said there is funding for a medical facility at a U.S. base in Germany that has been partially built. If those funds are used, the medical center could be left half built.

The aides said that while the president can decide to use military construction funds, it would likely be up to the Defense Department to determine which specific projects would lose their money.

WHAT WOULD HAPPEN THEN?

An emergency declaration is sure to spark a flood of lawsuits. Speaker Nancy Pelosi said Thursday the House of Representatives might sue, and some Democratic state attorneys general said their states would, too. Public and private landowners along the border also would be very likely to go to court if the administration tried to seize land for the wall through the process called eminent domain.

The legal fights would test the extent of the president’s authority as well as whether the situation at the border really constitutes an emergency.

Even if the president has the authority to move money around, the congressional aides said a key question will be if Trump can prove the funds are being spent in support of the military.

Trump critics concede that the president has broad authority to declare a national emergency under a 1978 law. But it’s not “a blank check to invoke ‘emergency’ powers simply because he couldn’t get what he wanted through the normal political process,” said Brianne Gorod, chief counsel of the liberal-leaning Constitutional Accountability Center.

But the Supreme Court has never overruled a presidential declaration, conservative scholar John Yoo said in a recent telephone briefing arranged by the Federalist Society.

Another issue that may confront a declaration is that border security is “thoroughly civilian,” David French, a senior writer at the conservative National Review, said on the same Federalist Society briefing. French said it’s a “stretch” to think that the military is needed at the border.

But he said that federal judges might be reluctant to get involved, especially because Congress has the authority under the National Emergencies Act to terminate a national emergency.

That prospect seems unlikely, however, now that Senate Majority Leader Mitch McConnell has said he will support Trump’s emergency declaration.