In 1976, Mao died, and in 1978, the Gang of Four were overthrown by Hua Guofeng, who was to soon implement pro-market economic reform. With the PRC no longer espousing the anti-revisionist notion of the antagonistic contradiction between classes, relations between the two countries became gradually normalized. In 1979, however, the PRC invaded Vietnam (which had, after a period of ambivalence, sided with the Soviet Union) in response to the Vietnamese invasion of Cambodia which overthrew the China-backed Khmer Rouge from power.