In the early 1980s, according to newly released documents, Fidel Castro was suggesting a Soviet nuclear strike against the United States, until Moscow dissuaded him by patiently explaining how the radioactive cloud resulting from such a strike would also devastate Cuba.

The cold war was then in one of its chilliest phases. President Ronald Reagan had begun a trillion-dollar arms buildup, called the Soviet Union “an evil empire” and ordered scores of atomic detonations under the Nevada desert as a means of developing new arms. Some Reagan aides talked of fighting and winning a nuclear war.

Dozens of books warned that Reagan’s policies threatened to end most life on earth. In June 1982, a million protesters gathered in Central Park.

Barack Obama, then an undergraduate at Columbia University, worried about the nuclear threat and later wrote as a student and a journalist about ways to avoid global annihilation.