Joseph Stalin was the leader of Soviet Union from the mid-1920s until his death in 1953.
Stalin was a very shrewd manipulator. Lenin had entrusted him with several posts that were seemingly of little importance; General Secretary of the Party for example.
Stalin used this post to control who was in - or out of the Party. Stalin then ensured that 'his' people were loyal to him because they wanted to keep their jobs - he did this by collecting information about everyone in the Party.
Once Lenin was dead Stalin was just as shrewd. He let other members of the Politburo present policies, and argue amongst themselves. The Politburo shot itself in the foot with the policy of Democratic Centralism - basically this meant that once the Politburo has discussed a problem & decided on a policy - there was to be no more debate about it.
Stalin then let everyone discuss programmes and problems - he then sided with the winning team & isolated the losers. He repeated this - isolating many of the Revolutionary Old Guard - until he was confident of success in removing them, through arrest, exile & murder.....