If in an emergency the President considers it urgent, a temporary fort or fortification may be built on private land if the owner consents in writing.
Structure US Code
PART IV— SERVICE, SUPPLY, AND PROCUREMENT
§ 7771. Acceptance of donations: land for mobilization, training, supply base, or aviation field
§ 7772. Heritage Center for the National Museum of the United States Army: development and operation
§ 7776. Emergency construction: fortifications
§ 7777. Permits: military reservations; landing ferries, erecting bridges, driving livestock
§ 7779. Use of public property
§ 7780. Acquisition of buildings in District of Columbia
§ 7781. Cyber Center for Education and Innovation-Home of the National Cryptologic Museum