Terrorists. Narcotrafficking. Coronavirus. Refugees. There are many reasons—real or imaginary, compelling or contrived—for governments to want to restrict people’s movement. In March 2019, President Trump hinted that he was considering closing the border with Mexico. He cited the vast numbers of migrants approaching the southern border as a justification for closure, then pivoted to concerns about drug trafficking. Possibly emboldened by a victory in the 2018 “travel ban” case of Trump v. Hawaii, the President asserted a unilateral power to close the border. The general consensus among political leaders and economists was that closing the border would be an economic and political catastrophe, disrupting billions of dollars’ worth of goods while doing little to combat the asylum backlog or illegal narcotics trafficking. He soon backed off, with a one-year warning to Mexico that is fast approaching as of the publication of this Note. But the question remains: Can he do it? This Note considers the question and concludes that while very brief or geographically limited closures are authorized as a matter of statute and constitutional doctrine, any indefinite, long-term, and expansive border closure would be statutorily unauthorized and give rise to meritorious due process claims by some categories of noncitizens. In between these two extremes, the permissibility of a closure would depend on the temporal and geographical scope of it, tracking general separation-of-powers principles.