Short answers, pulled from the story.
The Sino-Soviet border conflict was a seven-month undeclared military conflict between the Soviet Union and China in 1969. The most serious clashes occurred on Zhenbao Island in the Ussuri River in Manchuria in March 1969, with further fighting in Xinjiang in August 1969.
A scholarly consensus holds that the fighting was a premeditated act of aggression orchestrated by the Chinese side. Western historians attribute it to Mao Zedong's domestic political imperatives during the Cultural Revolution; Russian historians point to China's desire to raise its international standing and strengthen ties with the United States. The Soviet invasion of Czechoslovakia in August 1968 and the Brezhnev Doctrine also heightened Chinese fears of a Soviet attack.
By 1968, the Soviet Union had 375,000 men, 1,200 aircraft, and 120 medium-range missiles stationed at the Sino-Soviet border. China had 1.5 million troops on its side of the border.
In October 1969, the Soviet Union contemplated a massive nuclear strike on China. In August 1969, the CIA described the conflict as having "explosive potential" and noted the potential for war clearly existed, including a possible Soviet attack on Chinese nuclear facilities. China evacuated Beijing and relocated three top-secret nuclear production plants to interior regions.
The immediate crisis de-escalated after Zhou Enlai and Alexei Kosygin met at Beijing Airport on the 11th of September 1969 and agreed to return ambassadors and begin border negotiations. The underlying border disputes were not conclusively settled until a treaty signed on the 14th of October 2003 and an additional agreement signed on the 21st of July 2008.
Facing the Soviet threat, China sought a strategic counterbalance by pursuing rapprochement with the United States. Henry Kissinger made a secret visit to China in 1971 to meet with Zhou Enlai, which paved the way for President Richard Nixon's official visit to China in 1972, when Nixon met with Mao Zedong.