What event lead to the end of the cold war?
Hungary tearing down its border fence.
The reason this was the catalyst for everything afterward, is people who lived in the Warsaw Pact had freedom of movement within Pact nations, and Hungary was a popular vacation spot for Warsaw Pact citizens. By the time the communists figured out people weren't going there to lie on the beach - a very popular vacation for Warsaw Pact members - but to defect to the West, all their doctors and engineers were living in West Germany.
The Cold War stayed "cold" due to both the fear and nuclear power residing in each country. If the United States and the Soviets began an artillery based war it would eventually lead to a nuclear war; an event that could have potentially killed millions of people residing in both the United States and Russia.