A soviet civil war happens

During its existence, except for its beginning, the USSR was a relatively stable country - stable, let's be clear, in a sense that the political transitions never began any civil war or big conflict, with the political opponents being eliminated before they could make such a movement. So, what if there was a Civil war between the leadership of the party? The easiest way would probably be Stalin vs Trotsky, but there are any others?
 
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