The United Opposition was repeatedly threatened with sanctions by the Stalinist leadership of the Communist Party, and Trotsky had to agree to tactical retreats, mostly to preserve his alliance with Zinoviev and Kamenev. The opposition remained united against Stalin throughout 1926 and 1927, especially on the issue of the Chinese Revolution. The methods used by the Stalinists against the Opposition became more and more extreme. At the XV Party Conference in October 1926, Trotsky could barely speak because of interruptions and catcalls, and at the end of the Conference he lost his Politburo seat. In 1927, Stalin started 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.