Black helicopters is a term which became popular in the United States militia movement and associated political groups in the 1990s as a symbol and warning sign of an alleged conspiratorial military takeover of the United States, though it has also been associated with men in black and similar conspiracies. Rumors circulated that, for instance, the United Nations patrolled the US with unmarked black helicopters, or that federal agents used black helicopters to enforce wildlife laws.
Metonymic use of the phrase black helicopters sometimes occurs in reference to conspiracy theories in general.
Stories of black helicopters first appeared in the 1970s, and were linked to reports of cattle mutilation. It is possible that the idea originated in Hal Lindsey's book The Late, Great Planet Earth, published in 1970 and popular among conspiracy theorists. Lindsey theorized that the locust-like creatures referenced in the Book of Revelation were actually helicopters, which John had never seen and thus did not know how to describe.
Jim Keith wrote two books on the subject: Black Helicopters Over America: Strikeforce for the New World Order (1995), and Black Helicopters II: The End Game Strategy (1998).
Media attention to black helicopters increased in February 1995, when first-term Republican northern Idaho Representative Helen Chenoweth charged that armed federal agents were landing black helicopters on Idaho ranchers' property to enforce the Endangered Species Act. "I have never seen them," Chenoweth said in an interview in The New York Times. "But enough people in my district have become concerned that I can't just ignore it. We do have some proof."