After a series of successful defenses against Nationalist Army attacks, Chiang's German advisers switched tactics and began building concentric circles of fortified positions closer and closer to the communist base. This forced the party to embark in the famous Long March of October 1934 to October 1935. Shortly after the march began, party leaders held an enlarged congress to determine the direction and leadership of the revolution. At the Zunyi Conference in 1935, the 28 Bolsheviks were defeated by Mao Zedong and his allies, primarily due to the backing of Zhou Enlai and Zhu De.