Sean Davis has an excellent essay on the subject of President Trump's legal authority to declare the border situation a national emergency and use funds originally allocated by congress for other purposes to construct a wall along our southern border.
Davis breaks the issue into several questions which I list below with a summary of Davis' response to them:
Does President Trump have the legal authority to declare a national emergency along the border?
National Emergencies Act of 1976, explicitly authorizes the president to declare a national emergency. Congress put no constraints on whether a president may declare an emergency, or what conditions must be met in order for a particular event or crisis to be considered an “emergency.” Instead, the law leaves that decision solely up to the president.
Does the president have the legal authority to use funds allocated for other purposes for wall construction under the national emergency?
Within the context of the emergency border wall debate, that law is 10 U.S.C. 2808, which delegates to the president, in the event of a national emergency that requires the U.S. military, the authority to reprogram existing appropriations for military construction projects in order to address the ongoing emergency.
Who determines whether the use of the armed forces is required during a particular national emergency? The simple answer is that such discretion belongs to the president of the United States in the discharge of his duties as commander-in-chief.
Is a border wall a military construction project?
Some legal commentators assert that the answer is “no,” since immigration enforcement and border security generally fall under the jurisdiction of the Department of Homeland Security.
As a broad matter, that seems to be a difficult position to hold, given that it requires one to argue that the military has no role in protecting the United States. If border security is solely the matter of non-military agencies, then it must follow that the U.S. military would have no role, authority, or jurisdiction in repelling an invasion of the United States, a position that is clearly nonsensical.
Similarly, it makes little sense to argue that two presidents were justified in using emergency authority to reprogram funds to build and maintain U.S. facilities in foreign nations in order to protect their security, as they did 18 separate times between 2001 and 2014, but that the current president has no authority to use the same funds to secure actual American soil from foreign invasion.
But should he do it?
As a legal matter, Trump’s authority to declare a national emergency on the border then reprogram a limited amount of existing military construction or civil defense project appropriations to address the emergency is well-established. Whether it is wise to do so as a political matter is an entirely different question. The question of “should” is entirely different than the question of “can.” And there many valid political arguments on both sides of the debate.
Davis presents much more detail in his column, and it's worth reading regardless of whether one is in favor of or opposed to a border barrier.
Unfortunately, proper legal authority is not likely to be a primary consideration for activist judges who've demonstrated a willingness in the past to ignore the law in order to impose their own personal will on the elected president of the United States.
That's why most observers, including Mr. Trump, believe this controversy will only be resolved when it reaches the Supreme Court.