How did Stalin become the most powerful man in the Soviet Union?

 (My answer on Quora)

Stalin played the long game and he played it well. His rise to power was a combination of opportunism and ruthlessness. The skill set set that served him best was his ability to organize that he demonstrated in earnest during the Bolshevik uprising of the July Days in 1917. He also understood the power of media propaganda through his role as the principal editor of the Bolshevik's Central Committee's newspaper.

Stalin also had a aptitude for forging alliances with other or two other power brokers to isolate a potential rival (such as Trotsky). Once done with that marriage of convenience he would play his alliance partners against one another so that he emerged from the cesspool of power. He used his pawns in the middle game and finished with his queen.

To put it mildly if the man was a contestant on Survivor nobody would have stood a chance.