If you’ve been paying any attention to the news, you know that on February 15, 2019, President Donald Trump declared a national emergency along the U.S.-Mexico border, stating that he would use $3.6 billion of military funds to build a wall along the border. What gives him the authority to do this?

Broadly speaking, that authority arises from the National Emergencies Act of 1976, which Congress passed in order to formalize and define the limits of the President’s power to declare a national emergency. Previously, multiple presidents, starting with Woodrow Wilson, had declared national emergencies, some of which lasted for decades after being declared. Among the four ongoing national emergencies which the Act brought to an end in 1978 was President Truman’s 1950 proclamation calling upon the United States to fight Communism in Korea; the Korean war ended in 1953, but the national emergency remained in effect for another 25 years.

The powers the president can invoke under the Act are defined by law, though they’re not enumerated in the Act itself. Rather, other laws contain provisions granting the president certain powers in a national emergency, and the Act requires the president to specifically cite the law which authorizes his exercise of emergency power. There are over 130 laws containing emergency powers provisions, so the range of emergency powers the president can theoretically utilize are broad, but not unlimited. Since the passage of the Act, 59 national emergencies have been declared, of which 32 are still in effect; the majority of these emergency declarations have involved sanctions against foreign countries or violent organizations.

The most recent proclamation of a national emergency invokes two particular emergency powers. The first of these allows the President to call up to one million military reservists to active duty. The proclamation states that “The Secretary of Defense [. . .] shall order as many units or members of the Ready Reserve to active duty as the Secretary concerned, in the Secretary’s discretion, determines to be appropriate to assist and support the activities of the Secretary of Homeland Security at the southern border.” The proclamation fails to indicate exactly what sort of assistance or support the military is expected to provide to Homeland Security, and the deployment of troops to the border shortly before the 2018 elections don’t seem to have had any significant impact on border control.

The second provision is more directly related to the Trump administration’s desire for a border wall. The Secretary of Defense may undertake military construction projects during a national emergency, and the Trump administration clearly considers a border wall to be such a project. This law requires that the national emergency require the use of the armed forces and that the military construction project(s) are necessary to support the use of the armed forces. It also requires the construction to be funded from the Defense Department’s budget for military funds.

In practical terms, this allows the Trump administration to perform the following sleight of hand:

1. deploy the military to the border on a mission to “support” the Department of Homeland Security;
2. construct a border wall on the basis that it’s “necessary” to support that military deployment; and
3. reallocate money from the rest of the military’s construction budget to build that wall.

If building a border wall is a legitimate use of this emergency power, this would mean that so long as the President is willing to declare a national emergency, he can redirect any or all of the Department of Defense’s construction budget to the wall, even though Congress only authorized $1.375 billion for construction of a border barrier. If this use of emergency power holds up in court, it would mean that so long as a state of national emergency exists, the president can use the Department of Defense’s military construction budget as a slush fund for building his border wall or any other facility that he claims is “necessary” to support a military deployment.

Sixteen states, including Michigan, have already sued the federal government over the president’s declaration, calling it a “pretext” which, “contrary to the will of Congress,” is being used to “redirect federal dollars appropriated for drug interdiction, military construction and law enforcement initiatives toward building a wall on the United States-Mexico border.” According to the lawsuit’s pleadings, “The Michigan Department of Military and Veterans Affairs receives a majority of its funding from the federal government” and “it performs missions training and prepares citizen soldiers and airmen to respond to, among other things, state emergencies, military support, and protection of local communities. Loss of funding negatively impacts this vital service for the People of Michigan.”

If the state of national emergency lasts long enough for this lawsuit to go to trial, there are several issues the courts will have to rule on. One issue is separation of powers. The Constitution gives Congress responsibility for appropriations and budgeting, and Congress already budgeted $1.375 billion for border barrier construction. Additional money for a border wall would come from other projects which Congress has budgeted for, such as money allocated to Michigan’s Department of Military and Veterans Affairs. Does the president’s emergency power concerning military construction allow him to override Congress’s explicit decisions regarding the allocation of military funds, including funds earmarked for border barrier construction? Or do the emergency power provisions only allow the President to do so in situations where Congress has not already made a decision?

Another issue which might arise is whether or not a border wall would constitute “military construction projects” which are “necessary to support” the “use of the armed forces.” The law only grants the president emergency powers to construct facilities which meet those criteria; the president cannot siphon military funds to projects which don’t support the armed forces. Because of laws and regulations generally preventing the use of the military to enforce domestic law, prior military support to DHS for border protection has taken the form of logistical and medical support. If the military’s mission is to simply provide logistical support to DHS personnel, can a 2000 mile border wall plausibly be construed as “necessary’ to carrying out that mission? Or is a border wall a construction project intended to support the mission of the Department of Homeland Security, rather than that of the military? The answer to that question could determine whether the Trump administration can legally redirect money from the Department of Defense’s construction budget in order to build a wall.

President Trump’s declaration of a national emergency raises many questions about the proper uses of the president’s powers, and the extent to which the law constrains the president’s use of emergency powers. One thing which is clear, however, is that this declaration is on track to provoke a drawn-out legal and political battle. The declaration pits the federal government against state governments which face the prospect of having funds directed away from veterans’ affairs and emergency preparedness programs to fund a border wall. It pits the Executive branch of the federal government against the Legislative branch, as the Executive attempts to override the budget established by Congress by re-allocating military construction money to border barrier construction. It even pits Executive departments against each other, as the Department of Defense faces of possibility of seeing billions of dollars of its funding being spent on border barriers which appear to be intended to benefit the Department of Homeland Security, rather than the military.

Shortly after declaring the national emergency, President Trump said, “I could do the wall over a longer period of time. I didn’t need to do this [national emergency]. But I would rather do it much faster.” Ironically, by declaring a national emergency in a heavy-handed way which appears to both challenge Congress’s budgetary authority and threaten federal funding to individual states, leading to intense legal and political controversy, the president may have guaranteed that the federal government’s response to this alleged emergency will be anything but fast.