Warrantless searches are per se unreasonable under the Fourth Amendment, unless a court determines that the search is subject to an established exception. Few exceptions to the presumptive warrant and probable cause requirements of the Fourth Amendment are more firmly rooted than the border search exception. Authorized by the First Congress, the border search exception has a history as old as the Fourth Amendment and obtains its broad power from Congress’s authority to regulate commerce with foreign nations and to enforce immigration laws.