Short answers, pulled from the story.
The Sino-Soviet split was caused primarily by diverging interpretations of Marxism-Leninism, especially after Khrushchev's 1956 speech denouncing Stalin. Mao had modeled his leadership on Stalin and viewed de-Stalinization as revisionism that undermined both Soviet ideological authority and his own political legitimacy. Disputes over nuclear weapons sharing, peaceful coexistence with the West, and Soviet technical aid to India further deepened the rift.
The split began deteriorating in early 1956 following Khrushchev's de-Stalinization speech, with formal denunciations exchanged by 1961. The conflict reached its most dangerous point during the Zhenbao Island incident in 1969. Relations were gradually normalized through the 1980s and fully restored in 1989 when Mikhail Gorbachev visited China and met with Deng Xiaoping.
According to declassified sources from both the PRC and the United States, the Soviet Union planned a massive nuclear strike on China following the Zhenbao Island incident in March 1969. Soviet Defense Minister Andrei Grechko called for use of a multimegaton bomb. The plan was abandoned after the United States, according to a number of sources, informed the Soviets that it would launch nuclear attacks on approximately 130 Soviet cities in response.
The Sino-Soviet split transformed the bipolar Cold War between the US and USSR into a three-way rivalry among the US, USSR, and PRC. It enabled Sino-American rapprochement, culminating in Nixon's 1972 visit to China, and gave rise to the policy of triangular diplomacy. It also weakened the Western concept of monolithic communism, the belief that communist nations were ideologically unified.
The Zhenbao Island incident was a border clash in March 1969 along the Ussuri River between Chinese and Soviet forces. It was significant because it pushed the Sino-Soviet conflict from ideological dispute to open military confrontation, prompted Soviet planning for a nuclear strike on China, and triggered the mass evacuation of Chinese Communist Party leadership from Beijing in October 1969.
At the 1957 International Meeting of Communist and Workers Parties in Moscow, Mao said China would survive nuclear war even if half of its 600 million people were killed. Khrushchev recalled that the audience fell dead silent. The remarks alarmed communist leaders across Eastern Europe and helped convince the Soviet Union to renege on its 1957 commitment to provide China with a model nuclear bomb.