Creators.com: What’s Wrong with FISA? by Andrew P. Napolitano:
Congress enacted the Foreign Intelligence Surveillance Act in 1978 in response to the unlawful surveillance of Americans by the FBI and the CIA during the Watergate era. President Richard Nixon — who famously quipped after leaving office that “when the president does it, that means that it is not illegal” — used the FBI and the CIA to spy on his political opponents.
The stated reason was national security. Nixon claimed that foreign agents physically present in the U.S. agitated and aggravated his political opponents to produce the great public unrest in America in the late 1960s and early 1970s, and thus diminished Americans’ appetite for fighting the Vietnam War. There was, of course, no evidence to support that view, but the neocons in Congress and the military-industrial complex supported it even after Nixon left office.