In 1969, long-simmering Sino-Soviet tensions boiled over into direct military confrontation along the Ussuri River. The Ussuri served as an official border boundary between the People’s Republic of China and the Union of Soviet Socialist Republics, and it had been a point of Chinese contention that the Soviet Union had forced China to consign Chinese land to Tsarist Russia under the 1860 Treaty of Peking.
On March 2, Chinese troops issued attacks on Soviet personnel stationed along the border, to which the Soviets responded with attacks on PLA troops from the Zhenbao Island in Manchuria roughly two weeks later. This initiated a spiral of tit-for-tat exchanges along the Ussuri that lasted for several months during which increasing levels of firepower resulted in a considerable number of casualties on both sides.
Amid the conflict, leaders in the Soviet Union began issuing provocative threats of heightened military confrontation — including potential nuclear escalation — in hopes of coercing China to enter into negotiations for a peace settlement. When these initial threats did not have their intended effect, the Soviet Union began approaching foreign governments and petitioning for their reaction to a potential Soviet nuclear first-strike against China. Knowledge of the Soviet overtures convinced Chinese leaders of the credibility of their nuclear threats. China proceeded to raise the alert levels of their nascent nuclear forces in anticipation of a surprise nuclear attack, and by October Chinese leadership had fled the capital of Beijing. This represents the first and only time that China has placed its nuclear forces on full alert status.
The Soviets and the Chinese finally reached the negotiating table in October 1969 but would not reach a conclusive settlement for more than a decade. However, the re-opening of border negotiations marked the end of the nuclear crisis.