Most of the discussion of President Trump's proclamation of a national emergency has been focused on whether any emergency exists at the border and whether or by what standard the courts can review the existence of a national emergency. That raises issues of the scope of the president's executive power under Article II of the Constitution that have never been considered or decided by the federal courts. The leading Supreme Court decision on presidential emergency power, Youngstown Sheet & Tube Corp. v. Sawyer, assumed that President Truman's declaration of national emergency at the start of the Korean War was in effect. It held that the emergency did not give the president the power to seize the steel industry, in order to forestall a strike, when Congress had explicitly denied that power and enacted a less intrusive alternative when it passed the Taft-Hartley Act. The most influential aspect of Youngstown, Justice Jackson's concurrence, postulates a three-level hierarchy of emergency powers under Article II: when the president's action exercises power delegated by Congress (as was the case in Trump v. Hawaii), when Congress is silent, and when presidential action conflicts with Congress. In the last case, wrote Jackson, presidential emergency power is at its lowest ebb. If the Supreme Court follows Jackson, it may well conclude that the current proclamation does not authorize the president to spend to build border barriers beyond what Congress has already appropriated for that purpose.

The 1976 National Emergencies Act uses Jackson's Youngstown hierarchy to confine the president's emergency powers. It provides, first, that any declaration of national emergency must specify pre-existing statutes that grant emergency power, and then that only the emergency powers in the specified statutes may be exercised. 50 U.S.C. § 1631. Thus the act both grants emergency power in the incorporated statutes and denies emergency power beyond them. Any attempt to exercise broader powers would conflict with Congress's expressed will.

The current proclamation identifies only one statute, 10 U.S.C. § 2808, that gives the president the power to reallocate appropriated funds in an emergency. It provides that in a national emergency requiring the use of the armed forces, the secretary of defense “without regard to any other provision of law, may undertake military construction projects, and may authorize the Secretaries of the military departments to undertake military construction projects, not otherwise authorized by law that are necessary to support such use of the armed forces,” within the cap of the total appropriated for military construction. Section 2808 is not the key to the vault it first appears. Its definitions section, 10 U.S.C. § 2801 defines “military construction projects” as construction or land acquisition on “military installations.” It then defines “military installations” as “a base … or other activity under the jurisdiction of the Secretary of a military department.” In other words, § 2808 authorizes the emergency reprogramming of military construction funds to build facilities or buy land for existing military installations. Moreover, another statute, 10 U.S.C § 2664, prohibits the military from acquiring land for military installations “unless expressly authorized by law.” It is, to say the least, doubtful that § 2808 authorizes spending money to build barriers on land that is not part of an existing military base or to acquire land for new military bases. Most of the land along the border is not on military installations. If the courts read § 2808 as only authorizing emergency spending for construction on existing military installations, as the clear language of these statutes provides, they avoid the wider constitutional issues, and reduce the proclamation to a largely empty gesture, while staying firmly within the framework of the National Emergencies Act and Youngstown.