Prelude to the Great Purge | STALIN TURNS ON HIS ALLIES 

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After Stalin got rid of Trotsky, though Trotsky was still influencing Soviet politics through writing and publishing articles in exile, the troika between Lev Kamenev, Grigory Zinoviev and Stalin collapsed.
In the years leading up to and after Trotsky's exile, Stalin forms alliances with Zinoviev and Kamenev, only to turn on them by forming an alliance with Bukharin, Rykov and Tomsky. Though, Stalin realizes he does not need alliances but loyal followers. All of his former allies fall from grace and eventually end up expelled from the party. These years are only a prelude to their eventual definite elimination.
Song at the end: "Wide is my Motherland"/"Широка страна моя родная"
Timecodes:
1:11 Socialism in one country
3:01 Stalin turns on his allies
6:11 Prelude to the Great Purge
8:42 Next week (The Great Purge)
Sources:
Conquest, Robert. Stalin: Breaker of Nations (New York, 1991).
Hosking, Geoffrey A. Russia and the Russians: A History (Harvard University Press, 2010).
Pomper, Philip, and Phillip Bernhei. Lenin, Trotsky, Stalin (Columbia University Press, 1992).
Schlögel, Karl. Moscow, 1937 (John Wiley & Sons, 2014).
Video sources:
www.archive.org
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