By a number of unequal nineteenth-century treaties, Russia and the Chinese government agreed on a 4,500 mile-long border, running from the east through the northern border of modern Mongolia to the west of the PRC's Xinjiang Province. In the east, the demarcation line was drawn along the Amur River, known to the Chinese as Heilongjiang (literally meaning "the Black Dragon River"), along Heilongjiang Province's northern border, and along the Ussuri River, an Amur tributary between the eastern border of Heilongjiang and Russian Siberia. This border remained as it was after the fall of the Chinese Qing Dynasty and the Russian regime in the late 1910s and was twice reiterated, first with the Republic of China (ROC) in 1945 and then with the PRC in 1951.
Owing to its heavy reliance on the Soviet Union and the need to maintain Sino-Soviet solidarity, the PRC had consistently refrained from raising the border issue despite its nationalistic effort to eliminate the unequal treaties and restore territorial integrity. As the PRC sought to establish its independence from Soviet influence and the Sino-Soviet split loomed, the border issue surfaced in 1960 when the PRC revoked the unequal treaties. Border negotiations began in February 1964 in Beijing but broke off in October 1964 when the Soviets insisted on maintaining the status quo. Tensions subsequently mounted along the Sino-Soviet border, with increased armed incidents. By 1969, there were 658,000 Soviet troops confronting 814,000 Chinese troops along the Amur River. The Soviet Union had also secured the permission of the Mongolian government to base Soviet ground and air forces in that nation.
On 2 March 1969, a Soviet border patrol was ambushed by Chinese troops on Damansky Island in the Ussuri River, resulting in 31 Soviet deaths. On 15 March 1969, the Soviet Union retaliated by firing on Chinese troops on Damansky Island, causing nearly 800 deaths. Cascades of diplomatic protests and counterprotests and more border clashes followed. The most serious border incident occurred on 13 August 1969 on the border between China's Xinjiang Province and the Soviet Republic of Kazakhstan, pushing the PRC and the Soviet Union to the brink of nuclear confrontation. In response to the PRC's successful explosion of its first atomic bomb in October 1964, the Soviet Union had increased its nuclear forces in Asia, including those in the Democratic People's Republic of Korea (DPRK, North Korea) and Mongolia. By 1969, the Soviet Union had installed an antiballistic missile system directed against the PRC. The Soviet action on the Xinjiang border seemed to confirm the PRC's suspicion that the Soviets were planning a preemptive strike against Chinese nuclear installations in Xinjiang.
A war was averted when, on 11 September 1969, Soviet Premier Alexei Kosygin visited Beijing, meeting with Premier Zhou Enlai to settle the border conflict. Both sides agreed on three principles: keeping the status quo of the Sino-Soviet border, not employing military forces, and preventing future military clashes. On 20 October 1969, the border talks suspended in 1964 resumed in Beijing, with concentration on the Amur-Ussuri demarcation line.
These negotiations made little progress, however. The Soviet Union continued its military buildup along the border throughout the 1970s. By the end of the decade, the Soviet Union had tied down 25 percent of its conventional forces along the border. Perceiving this massive deployment as a threat, the PRC decided to normalize its relationship with the United States, culminating in the formal establishment of the Sino-U.S. diplomatic relationship in 1979.
Tensions along the Sino-Soviet border began to ease in July 1986, when Soviet leader Mikhail Gorbachev expressed his desire to restore Sino-Soviet harmony and settle the border disputes. In 1990, Soviet border forces were drastically reduced. Significant progress in the border talks was only achieved after the Cold War ended and the Soviet Union was dissolved. As a first step, Russia returned Zhenbao Island to the PRC on 19 May 1991. Finally, on 14 October 2004, both sides proclaimed the demarcation of the 4,500-mile Sino-Soviet boundary as complete and uncontested.
Lowell, Dittmer. Sino-Soviet Normalization and Its International Implications, 1945–1990. Seattle: University of Washington Press, 1992.; Wishnick, Elizabeth. Mending Fences: The Evolution of Moscow's China Policy, from Brezhnev to Yeltsin. Seattle: University of Washington Press, 2001.