A key turning point in Sino-Soviet relations was the thaw in relations between the USSR and the United States. This rapprochement angered Mao Zedong and helped to make the Sino-Soviet relationship somewhat hostile for decades.
In the mid-1950s, the PRC and the US came into serious conflict over the Taiwan Straits and Chinese bombardment of the islands of Quemoy and Matsu. The US threatened to use nuclear weapons against China. China felt that the USSR did not support it enough in this confrontation.
A few years later, Khrushchev, the Soviet leader, visited the US. This was seen by the Chinese as even more evidence that their supposed communist comrades were abandoning them in favor of peaceful coexistence with the West. Mao did not approve of this policy, wanting more confrontation and more communist solidarity. This led to increased hostility between the two communist countries.