Short answers, pulled from the story.
Nuclear warfare first occurred on the 6th of August 1945 when a uranium-type weapon codenamed Little Boy destroyed nearly 50,000 buildings and killed approximately 70,000 people in Hiroshima. This event marked the only wartime use of nuclear weapons until three days later when Fat Man struck Nagasaki.
The Soviet Union tested its first nuclear weapon at Semipalatinsk in Kazakhstan on the 29th of August 1949. The United Kingdom followed with its first independent atomic bomb test in 1952 while France developed its first atomic bomb in 1960 and China did so in 1964.
During the Cuban Missile Crisis in 1962 the Soviet Union placed medium-range ballistic missiles in Cuba as a direct response to American Jupiter missiles in Turkey. After intense negotiations the Soviets removed their missiles from Cuba and the United States secretly dismantled its launch sites in Turkey over two decades later.
By 1980 there were about 40,000 nuclear warheads in existence globally with a potential combined explosive yield of approximately 13,000 megatons. These weapons had a potential combined explosive yield that represented a fraction of natural geological forces but remained capable of catastrophic destruction.
Stanislav Petrov prevented World War III on the 26th of September 1983 when he positively identified an incoming American first strike warning as a false alarm and did not report it to his superiors. His actions may have prevented World War III since Soviet policy required immediate nuclear response upon discovering inbound ballistic missiles.