The deceptively simple question to be answered in this essay is: how did Stalin rule? How did he maintain his authority while establishing a personal autocracy? His extraordinary and brutal political achievement was to act in the name of the Communist party and its central committee against that party and central committee, while remaining the unchallenged head of party and state and, evidently, a vastly popular leader. At the end of the process his absolute grip on power allowed him to declare black white and completely reverse the foreign policy of the Soviet Union and the line of the Comintern by embracing Nazi Germany in a non-aggression pact. The colossal and costly destruction he brought upon the country on the eve and in the early days of the Second World War gave rise to no organised opposition, and the centralised apparatus of control that he had created was not only able to weather the Nazi invasion but to organise a victory that would preserve the essence of the system he forged for another half-century.
The simplest, though inadequate, answer to the question, would be that Stalin's power was maintained through the exercise of terror and monopolistic control of the means of communication throughout society. Though certainly an important part of the answer, an exclusive focus on terror and propaganda does not explain how Stalin won his authority within the party in the 1920s and maintained it among his own supporters even before the advent of the Great Terror.