A nuclear weapons command exercise by NATO in November 1983 prompted fear in the leadership of the Soviet Union that the maneuvers were a cover for a nuclear surprise attack by the United States, triggering a series of unparalleled Soviet military responses, according to a top-secret U.S. intelligence review that has just been declassified. “In 1983, we may have inadvertently placed our relations with the Soviet Union on a hair trigger,” the review concluded.