Operation Sky Shield, properly Exercise Sky Shield,[1] was a series of three large-scale military exercises conducted in the United States and The United Kingdom in 1960, 1961, and 1962 by NORAD (North American Air Defense) Command and CONAD (Continental Air Defense) Command to test defenses against an air attack from the Soviet Union. The tests were intended to ensure that any attacks over the American–Canadian border or coastlines would be detected and then stopped.
The exercises involved 6,000 sorties flown by aircraft of the United States Air Force and Royal Air Force (RAF) (1961 only), simulating Soviet fighter and bomber attacks against New York, Chicago, San Diego, Los Angeles, Washington and more. They were among the largest military aviation exercises ever held.[citation needed]
The United States assured citizens that their defenses were "99 percent effective",[2] but the results showed how unsuccessful the defense would be against a Soviet air attack. No more than one quarter of bombers in Sky Shield would have been intercepted, according to later reports.[3] The results of the tests were classified until 1997 over fears that they could be used by the Soviets to engage the US more effectively in the event of World War III.
In the exercises, all air traffic from the Arctic Circle to Mexico was grounded, sometimes for up to twelve hours. The estimated cost of the shut downs was millions of dollars. In the reporting of the September 11 attacks in 2001, these exercises were often overlooked, with news agencies reporting that the similar but unplanned evacuation of US airspace during that incident had been first ever clearing of US airspace of all civilian aircraft.