 Like 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.After arriving back in Shanghai in 1929, Zhou began to work underground, establishing and overseeing a network of independent Communist cells.Zhou's greatest danger in his underground work was the threat of being discovered by the KMT secret police, which had been established in 1928 with the specific mission of identifying and eliminating Communists.