The Soviet Union secretly advanced some of their nuclear arsenal into parts of Cuba. Soviet long range intercontinental ballistic missiles or ICBM's that were placed in Cuba were well within range to hit nearly any part of the United States with a nuclear blast. The United States received intelligence from spy planes of the nuclear silos and thus had 13 days of strategic "no shots fired" conflict to try and get the missiles out of Cuba.
This conflict seriously did threaten global nuclear war. It was one of the highest tension periods in world history. Under the concept of "mutually assured destruction" if the Soviet Union was to have stuck the United States with a nuclear missile from Cuba, the US would strike back even harder, and so on and so forth.
