The FBI processed for release a 2054 page file dating from the mid-1950s describing how the world nearly ended. However, the below pages are the only releasable copies.
The File is entitled JOINT CHIEFS OF STAFF, JOINT US-CANADIAN ESTIMATE RE SOVIET CAPABILITIES AND PROBABLE COURSES OF ACTION AGAINST NORTH AMERICA IN A MAJOR WAR. The file number is FBI File No. 62-HQ-101831, and the file was sequestered for decades in the FBI Confidential File Room.
One interesting document within the file is a study issued by the United States-Canadian Intelligence Conference, ACAI 44, on September 11, 1957.
The study is Soviet Capabilities and Probable Courses of Action Against North America in a Major War Commencing in Mid-1961.
They were close in their assumptions: the Bay of Pigs Invasion of Cuba occurred in April 1961, JFK urged Americans to build fallout shelters in July 1961, the Berlin Wall began construction in August 1961, and the Cuban Missile Crisis took place in October 1962.
Soviet Capabilities and Probable Courses of Action Against North America in a Major War FBI File [20 Pages, 6743 kb] – These were just the pre-processed pages released by the FBI. I have filed an MDR request on April 13, 2021, to see if additional pages can be released.