To mark the 60th anniversary of the Korean War, the CIA earlier this year released 1,300 documents from 1950-1953 outlining, among other things, extensive planning by the United States to use nuclear weapons against North Korea during the Korean War, as well as threats to do the same.
According to an Associated Press summary of the documents,
Based on previously declassified documents, however, historians believe the U.S. came closest to unleashing its atomic arsenal against North Korea in April 1951, on the eve of an expected Chinese offensive.
With Truman’s signoff, the Joint Chiefs of Staff ordered A-bomb retaliation if large numbers of fresh Chinese troops entered the fight. In the end, the U.S. military repelled the Chinese push and the weapons were never used. But Pentagon planners retained the option.
The planning went so far as to include rehearsal raids on Pyongyang with B-29s that involved dropping dummy nuclear weapons on the city.
The newly released documents are available on the CIA’s website here.