Explain the break-up of the soviet union. why and how did it happen?

Officially the break-up of the Soviet Union ocurred on December 26 1991. However, this incredible event started to happen when Michail Gorbachev was elected general secretary by the Politburo on March 11 1985. Gorbachev was a man with good intentions and he stablished a policy of glasnot in Russian which means transparency and that was enough for people to realize that the central planned economy and this big union was not working for them. The rest is history, but Gorbachev was the key  for the change in my humble opinion.

c. as a force against soviet invasion of afghanistan