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The Sixth Party Congress

After the failure of the Nanchang Uprising, Zhou left China for the Soviet Union to attend the Chinese Communist Party's (CCP) Sixth National Party Congress in Moscow, in June–July 1928. The Sixth Congress had to be held in Moscow because conditions in China were considered dangerous. KMT control was so tight that many Chinese delegates attending the Sixth Congress were forced to travel in disguise: Zhou himself was disguised as an antiquarian.

At the Sixth Congress, Zhou delivered a long speech insisting that conditions in China were unfavorable for immediate revolution, and that the main task of the CCP should be to develop revolutionary momentum by winning over the support of the masses in the countryside and establishing a Soviet regime in southern China, similar to the one that Mao Zedong and Zhu De were already establishing around Jiangxi. The Congress generally accepted Zhou's assessment as accurate. Xiang Zhongfa was made secretary general of the Party, but was soon found incapable of fulfilling his role, so Zhou emerged as the de facto leader of the CCP. Zhou was only thirty years old.

During the Sixth Congress, Zhou was elected Director of the Central Committee Organization Department. His ally, Li Lisan, took over propaganda work. Zhou finally returned to China, after more than a year abroad, in 1929. At the Sixth Congress in Moscow, Zhou had given figures indicating that, by 1928, fewer than 32,000 union members remained who were loyal to the Communists, and that only ten percent of Party members were proletarians. By 1929, only three percent of the Party were proletarians.

In early 1930, Zhou began to disagree with the timing of Li Lisan's strategy of favoring rich peasants and concentrating military forces for attacks on urban centers. Zhou did not openly break with these more orthodox notions, and even tried to implement them later, in 1931, in Jiangxi. When the Soviet agent Pavel Mif arrived in Shanghai to lead the Comintern in China in December 1930, Mif criticized Li's strategy as "left adventurism", and criticized Zhou for compromising with Li. Zhou "acknowledged" his mistakes in compromising with Li in January 1931 and offered to resign from the Politburo, but was retained while other senior CCP leaders, including Li Lisan and Qu Qiubai, were removed. As Mao later recognized, Mif understood that Zhou's services as Party leader were indispensable, and that Zhou would willingly cooperate with whoever was holding power.