A government shutdown that lasted 35 days did not get U.S. President Donald Trump the $5.7 billion he wants for a wall on the Mexico border. In the wake of a deal that re-opened the government temporarily, White House aides said that if Democrats wouldn’t support a wall Trump might bypass Congress entirely by declaring a national emergency. When congressional negotiators reached a compromise longer-term deal that also didn’t give him the wall money he wanted, Trump said that he would sign the bill but accompany it with an emergency declaration, which he did on Friday. It’s an idea that’s drawn opposition from some Republicans as well as Democrats.

1. What exactly is a national emergency?

It’s a declaration by the president that gives him special, temporary power to deal with a crisis. In the past, most such invocations have been related to foreign policy, like prosecuting a war or responding to a global trade threat. International concerns explain most of the 28 currently active national emergencies. On a few occasions, however, presidents have used emergency declarations to further their domestic policy goals.

2. How would an emergency declaration help Trump?

In theory, it would let him redirect federal money allocated for other purposes and use it to at least get construction started on the wall he wants to build on the border with Mexico. Under the law governing the Pentagon, for instance, if the president declares a national emergency, the defense secretary is allowed to redirect money from military construction funds for projects “necessary” to support the deployment of U.S. armed forces. Or the defense secretary could terminate or defer the construction of Army civil works projects and apply those funds to “authorized civil works, military construction, and civil defense projects that are essential to the national defense.” Another option that’s been discussed: shifting funds from a disaster-spending bill passed by Congress that includes $13.9 billion in not-yet-spent money.

3. Would that free up enough money to build the wall?

The president’s request for $5.7 billion in wall funding represents more than half of the $10.3 billion appropriated for military construction this year and would mean sidelining politically popular projects. That could particularly hit military families, with the government slated to spend $1.6 billion of that budget on family housing, $352 million for medical facilities, and $267 million on educational facilities.

4. Does this really qualify as an emergency?

There’s a case to be made that it’s entirely up to the president to decide whether it does or not. Emergencies have been declared during crises large (the Civil War, the Great Depression, the aftermath of the Sept. 11 terror attacks) and small (remember the 1970 postal strike?). In 1976, Congress adopted a law giving the president broad authority to declare a national emergency. But the law, passed as part of a sweeping set of legislation designed to restrain presidential powers after the Watergate scandal, also demands that the president invoke specific statutory authority for emergency actions.

5. Would an emergency declaration be challenged?

Count on it. Congressional Democrats have already said they would mount a court challenge to any use of emergency authority. The Congressional Research Service, in a Jan. 10 report, said invoking a national emergency for this purpose “would raise a variety of novel legal issues,” including whether Trump is addressing a problem that really “requires use of the armed forces” and whether the planned wall would really “support such use of the armed forces.” During the initial shutdown, Trump said both that he has “the absolute right to do it,” but acknowledged that the final decision would likely be up to the Supreme Court.

6. What’s happened in previous fights?

Courts have limited the president’s emergency powers before, including in a 1952 Supreme Court decision in which the high court said President Harry Truman could not use an emergency declaration to seize steel factories to ensure production during the Korean War. But since laws governing emergency authority have changed, some experts say it would be difficult to predict how the case would be viewed by the current Supreme Court, in which Republican-appointed justices hold the majority.