The Second World War was the bloodiest in human history. Raging on from 1939 until 1945, over 70,000,000 people lost their lives, and it ended with a catastrophic new weapon being used twice on Japan. The Allied nuclear bombings of the cities of Hiroshima and Nagasaki on 6 August and 9 August 1944, respectively, changed the face of war forever. It remains the only time nuclear weapons have been used in warfare, but after those dark days in Japan, the threat has always been present.
After World War Two, the world was politically halved between the Capitalist West and the Communist East. This marked the beginning of the Cold War. With America harbouring the know-how to create atomic bombs containing between 0.2 kilotons of TNT and 48 megatons, the Soviet Union quickly discovered its own methods for manufacturing weapons of mass destruction. Their efforts surpassed the trials of the US, and in 1961, the USSR tested the Tsar Bomba, which upon detonation released a yield of 50 megatons of TNT, with a potential of reaching up to 100. The threat from both sides was real.
With the two superstates having access to nuclear weapons, this prevented the Cold War from becoming a true, physical battle. If one side were to nuke the other, then retaliation would have been imminent, creating a stasis of mutually assured destruction.
The Cold War ended after the fall of the Berlin Wall in 1989. By the mid-'90s, the Strategic Arms Reduction Treaties (START) was underway in an attempt to dismantle the supermassive stockpile of nuclear weapons belonging to Russia and America.