Intelligence is essential on the domestic side as well, where law enforcement is the president's main function. Yet not until 1967 did the Supreme Court require warrants for electronic surveillance. Congress codified that a year later. But both the court and Congress expressly exempted foreign-intelligence gathering from the warrant requirement.

Unfortunately, the exception was not to last. Following the Vietnam War, Congress increasingly inserted itself into foreign affairs, as with the 1973 War Powers Act. With the Foreign Intelligence Surveillance Act, passed in 1978, Congress began micromanaging foreign intelligence gathering. That produced the "wall" between foreign and domestic intelligence gathering — with foreign-intelligence agents focused on security, and domestic agents on prosecution and hence on obtaining "admissible" evidence. Neither side talked to the other. Many believe the resulting communications failures played a role in 9/11