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In page Leon Trotsky:

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The United Opposition faced repeated threats of sanctions from the Stalinist leadership. Trotsky had to agree to tactical retreats, mainly to preserve his alliance with Zinoviev and Kamenev. The opposition remained united against Stalin throughout 1926 and 1927, especially regarding the Shanghai massacre. Stalinist methods against the Opposition became increasingly extreme. At the XVth Party Conference in October 1926, Trotsky could barely speak due to interruptions and catcalls; at its end, he lost his Politburo seat. In 1927, Stalin began using the GPU (Soviet secret police) to infiltrate and discredit the opposition. Rank-and-file oppositionists were increasingly harassed, sometimes expelled from the Party, and even arrested.[citation needed]