During the Cold War, P.R.C.-U.S.S.R relations deteriorated quickly. Deng Xiaoping played an important role in this process. Mao Zedong took Deng as one of his main successors, and tried to cultivate and test Deng by ordering him to deal with P.R.C.-U.S.S.R relations. Deng took a tough stance toward the U.S.S.R not only because of patriotic and ideological factors, but also the structural constraints of power, and the desire to accommodate himself to Mao's expectations. This was considered the best way to express his loyalty to Mao. In fact, Deng's performance on the relationship with the U.S.S.R. was appreciated and praised by Mao, and it's an important reason for Mao permitting Deng to return to power later. Deng's role here has many implications for ”Deng Xiaoping Studies”, including Mao-Deng relations, Deng's historical responsibilities, his influence on foreign policies and his human interactions.