Answer:
The cuban missile crisis
Step-by-step explanation:
when the Cuban Missile crisis started in 1962, the Soviet Union put missiles on Cuban lands which led America to go to Defcon 2 (defcon is the United states ranking of the amount of readiness required for a potential nuclear attack; Defcon 1 means Mutually assured destruction or M.A.D) it was the closest the world has been to starting a nuclear war.
(P.S) I hope this explains it