He was able to get into politburo very young because he was a good administrator and he had powerful patrons. But he became the leader of the USSR just because there was nobody else. Politburo was so old that everybody ahead of him, or more power hungry than him was in hospice or otherwise not fully functioning.
Then he started finally some of the common sense reforms needed.
His intention was never to drop communism or let the soviet block to disintegrate, but things got out of hand. His greatest act was let it happen even when it was against everything he had worked for.
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