Workers break ground on border wall construction on Aug. 23 about 20 miles west of Santa Teresa, N.M. (Cedar Attanasio/AP)

We learned this week that nearly $2 billion will be redirected from planned projects at military facilities in the United States and its territories to go to the construction of a wall on the border with Mexico. A bit less than $1 billion of that will come from the contiguous United States, including $160 million diverted from proposed construction at the U.S. Military Academy at West Point. An additional $33 million would have gone to an elementary school at Fort Bragg, N.C.; instead, it will be put toward building President Trump’s long-promised wall.

(Philip Bump/The Washington Post)

The ability to reallocate money from the Defense Department to wall construction is contingent on the national emergency that Trump declared in February, focused on a surge in migrants arriving at the U.S.-Mexico border. That declaration allowed the government to use an arcane rule to tap into defense funding, bypassing a Congress unwilling to appropriate the money.

“The problem of large-scale unlawful migration through the southern border is long-standing, and despite the executive branch’s exercise of existing statutory authorities, the situation has worsened in certain respects in recent years,” February’s formal declaration of emergency read. “In particular, recent years have seen sharp increases in the number of family units entering and seeking entry to the United States and an inability to provide detention space for many of these aliens while their removal proceedings are pending."

In the month that the national emergency was declared, there were about 67,000 apprehensions on the border with Mexico, 36,000 of which involved members of family units — parents and children, mostly. That was higher than the number of apprehensions in either of the first two years of Trump’s presidency, though it was broadly in line with apprehensions at the start of Barack Obama’s presidency. Over the next few months, though, apprehensions soared, hitting 144,000 in May.

(Philip Bump/The Washington Post)

But then apprehensions dropped. In July, the most recent month for which data is available, there were about 43,000 family-unit apprehensions and 72,000 in total — not dramatically different from February’s numbers.

It was also not an exceptional figure historically. The number of apprehensions in July 2019 was lower than the same month in each year from 2000 to 2006.

(Philip Bump/The Washington Post)

It’s important to note that 2006 was the year that Congress passed the Secure Fence Act, authorizing the construction of hundreds of miles of new barriers on the U.S.-Mexico border. That law and the subsequent construction helped push immigration to authorized entry points.

But the recent drop-off in apprehensions raises an important question: When does Trump’s national emergency end? In the abstract, that question centers on the proximate causes cited by the president: If apprehensions fall, is the crisis over? But in practical terms, the answer is direct: It ends when Trump says it ends.

Since the implementation of the National Emergencies Act in 1976, there have been nearly 60 declared emergencies. Most deal with export controls or terrorism; most have also been in effect for more than a decade. The graph below shows the duration of each existent emergency as of January (meaning that Trump’s border emergency isn’t represented).

(Philip Bump/The Washington Post)

Why do those emergencies persist? In part because Congress has hardly ever exercised its mandated oversight of the process. Lawmakers are supposed to review each national emergency every six months, but they have done so on only three occasions — out of more than 1,100 reviews that should have been conducted.

One of those three occasions was earlier this year, when House Democrats forced a vote on Trump’s border emergency. That vote passed the House and, thanks to 12 Republican defections, the Senate as well. There was just one catch: Halting the emergency necessitated Trump’s signature, which he declined to provide. There were not enough votes in Congress to overturn his veto.

That in itself answers the question of when the national emergency will end. It exists only because Trump was unable to obtain funding through Congress for construction of a wall. The announcement that the president was declaring an emergency overlapped with his approval of a budget agreement that he had held up in the hope that Democrats would cave on wall funding. At the time we pointed out that constructing a wall wasn’t going to be a quick response to the purported crisis. Of 300 eminent-domain fights that arose as the government sought land to build barriers after that 2006 law, more than a quarter were ongoing a decade later.

The emergency has never primarily been about apprehensions or migration. It is mainly about building the wall, which was so central to Trump’s 2016 campaign pitch. The emergency, then, will end once the wall is built — or once Trump is out of office.