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.