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Military strategist and intelligence chief

Following the failure of negotiations, the Chinese Civil War resumed in earnest. Zhou turned his focus from diplomatic to military affairs, while retaining a senior interest in intelligence work. Zhou worked directly under Mao as his chief aide, as the vice chairman of the Military Commission of the Central Committee, and as the general chief of staff. As the head of the Urban Work Committee of the Central Committee, an agency established to coordinate work inside KMT-controlled areas, Zhou continued to direct underground activities.

A superior force of Nationalist troops captured Yan'an in March 1947, but Zhou's intelligence agents (primarily Xiong Xianghui) were able to provide Yan'an's commanding general, Peng Dehuai, with details of the KMT army's troop strength, distribution, positions, air cover, and dates of deployment. This intelligence allowed Communist forces to avoid major battles and to engage Nationalist forces in a protracted campaign of guerrilla warfare that eventually led to Peng achieving a series of major victories. By February 1948 over half the KMT troops in the northwest were either defeated or exhausted. On 4 May 1948, Peng captured 40,000 army uniforms and over a million pieces of artillery. By January 1949, Communist forces seized Beijing and Tianjin and were firmly in control of north China.