Mikhail Gorbachev and the collapse of the Soviet Union

It’s 30 years since the fall of the Iron Curtain and the end of the Soviet Union. Mikhail Gorbachev inherited a country that had many problems but was not on the verge of collapse. Six years later, he ended the Cold War and allowed the break-up of the USSR. What were the social, political and global forces that led this result? And how did the hero of glasnost end up 

being regarded a traitor by so many in his home country?