It's 20 years since Kremlin hardliners attempted to overthrow Soviet leader Mikhail Gorbachev and halt his programme of political and social change. Here Bridget Kendall, BBC Moscow correspondent at the time, asks how this product of Soviet Communism ended up as its destroyer.
By his own admission Mikhail Gorbachev did not start out a reformer. He was staunchly patriotic and a committed Communist.
"I was a loyal citizen," he says with an ironic smile.
"And proof of that is that I chose Stalin as my special topic for my final school exams. The title I chose was 'Stalin is our Glory in Battle, Stalin is the wings of our Youth.' No-one dragged me into joining the Communist Party."
It was only when he moved to Moscow University in 1950 that he says he began to have doubts.
The death of Stalin in March 1953 liberated the intellectual atmosphere. And he made student friends who were to shape his later thinking, among them a Czech student called Zdenek Mlynar.
"Zdenek was my dearest friend. He was closer to me than anyone else, Soviet or otherwise," says Gorbachev. "He was so clever, and it was my luck to be friends with him. Our views coincided on many points."
All the more interesting, then, that Zdenek Mlynar went on to become the architect of the Prague Spring of 1967, the Czech experiment to "give socialism a human face".
By then Gorbachev had returned to his native Stavropol province in southern Russia and was a rising star in the regional Communist party. Mlynar went to visit him.
"It seemed to me he understood very well what I was talking about," said Mlynar in an interview before he died in 1997. "But he said that while it might work in Czechoslovakia, it wouldn't work in the Soviet Union."
In 1968 Soviet tanks rolled into Prague to crush the uprising, and a new wave of repression started in the Soviet Union as well.
Party members were ordered to defend the invasion and denounce anyone who questioned it. And Gorbachev, in his role as the loyal apparatchik, not only obediently complied, but says he took the party line at face value. An extraordinary feat of mental gymnastics, given that his Czech friend's reforms were being pulverised.
The next year, when he joined an official delegation visiting Czechoslovakia, his private conscience got the better of him.
"We arrived and it was shocking," he says. "People didn't want to talk to us. We'd been told they wanted our support, including the military action. But it wasn't true. It was disinformation. We visited one factory and people turned their backs on us. It really hit me hard."
No doubt in any police state where conversations can be bugged or reported, people practice the art of double-think.
Gorbachev seems to have mastered it to perfection - critical in private, but in public a model of energetic subservience, a perfect heir apparent for the Kremlin old guard.
From 1968 there was still a long road to go before Gorbachev made it all the way up the party ladder to manoeuvre his way into the top job in the Kremlin in 1985.
Yet all along it seems, he did not forget the lessons of the Prague Spring. Going too far too fast, he concluded, was a open invitation for a backlash.
And this was why, he argues, his own first perestroika slogans were exhortations to "accelerate" and "perfect" the existing system, not a headlong rush for change.
That came later, after he had lost control, and an attempted hardline coup on 20 August 1991 led to the end of the Soviet Union.
Those who say he was too cautious and indecisive are wrong, he now argues.
"In some ways I was late, but in some ways I wasn't slow enough," he insists.
Twenty years since his downfall, Gorbachev is applauded for helping to end the Cold War, but he has had to weather plenty of criticism - for failing to keep pace with the reforms he unleashed, for failing to dump the Communist Party sooner, and for turning into a victim of the compromises he once used to seesaw between supporters and opponents.
But it is still worth dwelling on how this dissident from within emerged, a product crafted by the system, who somehow found the steel and guile to dare to strike the first blow that would bring down the entire, monstrous Soviet edifice.
Gorbachev: The Great Dissident can be seen on BBC World News at 0930 and 2130 GMT on Saturday 13 August (Check for local screening times)