[I]n 1977, [President] Carter and his attorney general, 
    Griffin B. Bell, authorized warrantless electronic 
    surveillance used in the conviction of two men for spying 
    on behalf of Vietnam. 

    The men, Truong Dinh Hung and Ronald Louis Humphrey, 
    challenged their espionage convictions to the U.S. Court 
    of Appeals for the 4th Circuit, which unanimously ruled 
    that the warrantless searches did not violate the men's 
    rights. 

    In its opinion, the court said the executive branch has 
    the "inherent authority" to wiretap enemies such as terror
    plotters and is excused from obtaining warrants when 
    surveillance is "conducted 'primarily' for foreign 
    intelligence reasons."