With airports jammed with angry passengers, Republican senators blaming each other behind closed doors for the government shutdown, and President Trump’s poll numbers tanking, the writing was on the wall. Feeling pressure from his fellow Republicans to reopen the government, Trump announced Friday afternoon that a deal was reached for federal employees to come back to work as negotiations on border security are given a little more time.

A week ago, the White House was firm on its demand: Parts of the government will remain shuttered until Democratic lawmakers write a $5.7 billion check for a border wall. Trump’s capitulation (and make no mistake, it was a capitulation) is a complete reversal of the White House position and a blowout win for Speaker Nancy Pelosi, D-Calif., in her first month back on the job.

Trump, however, remained defiant. At the same time he asked lawmakers to work in unison for the benefit of the nation, he threatened to use the nuclear option if Congress can’t come up with a solution. "I have a very powerful alternative, but I did not want to use it at this time," Trump said in the White House Rose Garden. "Hopefully, it will be unnecessary.”

That "very powerful alternative” Trump is referring to is a declaration of national emergency, a proclamation that would provide him with a way to get his border wall money without having to go through the normal legislative process. Ordinarily, the executive branch is constitutionally barred from spending any taxpayer money on any program unless Congress explicitly authorizes and appropriates the funds. Outside of declaring war or authorizing the use of military force, the ability to appropriate funds, or not, is the legislative branch’s most coveted power.

Lawmakers from both political parties protect the power of the purse with every fiber of their being because it’s one of the few tactics Congress can employ to pressure the president. Trump’s declaring of a national emergency would rip that power away. If lawyers in the executive branch can argue that there is indeed a dire national emergency along the southwestern border with Mexico, billions of dollars in the military construction budget will be made available for the border structure Trump so desperately wants. Trump could task the Army Corps of Engineers to start building right away.

Democrats, who are as strongly opposed to a border wall as Trump is enamored by it, would not be powerless bystanders if the president tried to do an end-run around Congress. They could technically prohibit any money from being used for construction of a border barrier through legislation, although GOP opposition would likely kill it. They could file a lawsuit against Trump, arguing that his emergency declaration is not, in fact, an emergency, but rather an instigation of an artificial crisis in order to justify an unjustifiable project.

A lawsuit would work itself through the court system and wind up in the Supreme Court, where the case could prove to be one of the most important trials of executive power since the fight over military commissions in the George W. Bush era. The border wall could become a proxy war between the executive and legislative branches, a classic battle over constitutional power that the judiciary would have to arbitrate. If Trump loses that fight, the precedent it would set would impact the flexibility of presidents in the future to tap into their emergency powers.

Trump may decide that a national emergency is too big of a step. The House Freedom Caucus, Trump’s most reliable support base on Capitol Hill, has already questioned whether an emergency proclamation is a smart idea. Lawmakers such as Freedom Caucus Chairman Mark Meadows are concerned that a declaration now could provide a future Democratic president with the power to do something similar on progressive priorities like climate change. The White House may gamble on another government shutdown, perhaps believing that the public would blame Democrats for being too obstructionist this time around.

Either way, the three-week government reopening shouldn’t be celebrated as a major breakthrough. The end of the longest shutdown in history may turn out to be the intermission to the meatier second act.

Daniel DePetris (@DanDePetris) is a contributor to the Washington Examiner's Beltway Confidential blog. His opinions are his own.