He first belonged to the left wing of the KPD, led by Ruth Fischer. In 1923, he aligned himself with Arthur Ewert and Gerhart Eisler and became the political leader of the party's Mecklenburg district. He participated in the Hamburg Uprising and in 1924, had to flee to Vienna, from where he was expelled to the Soviet Union in 1925. There, he succeeded Ivan Katz as Communist Party representative to the Comintern. From July to December 1927, he represented the Comintern in China. Working with Georgian communist Vissarion Lominadze, he helped Chinese communists to organize the Guangzhou Uprising on 11 December 1927. The rebellion was a complete failure and resulted in great casualties. Chinese communist leader Zhang Guotao blamed Neumann for this, claiming that the German had insisted that Guangzhou should be held at all cost against a National Revolutionary Army counter-offensive although this was not feasible for the local communists.