The most serious border clash, which brought the world's two largest socialist states to the brink of war, occurred near Damansky (Zhenbao) Island on the Ussuri (Wusuli) River in Manchuria.

In 1964, the Chinese revisited the matter of the Sino-Soviet border demarcated in the 19th century, originally imposed upon the Qing dynasty by the Russian Empire by way of unequal treaties.

Moscow would not accept that interpretation, but by 1964, both sides had reached a preliminary agreement on the eastern section of the border, including Zhenbao Island, which would be handed over to China.

[13][unreliable source] In July 1964, CCP chairman Mao Zedong, in a meeting with the Japanese Socialist Party delegation, stated that Russia had unilaterally incorporated vast territories in Siberia and the Far East as far as Kamchatka.

A nightmare vision of invasion by millions of Chinese made the Soviet leaders almost frantic: "Despite our overwhelming superiority in weaponry, it would not be easy for the USSR to cope with an assault of this magnitude".

[7] Concerns about Chinese manpower and its people's war strategy ran so deep that some bureaucrats in Moscow argued that the only way to defend against a massive conventional onslaught was to use nuclear weapons.

By threatening to initiate a prolonged conventional conflict in retaliation for a nuclear strike, Beijing used an asymmetric deterrence strategy that was intended to convince Moscow that the costs of an attack would outweigh the benefits.

Speaking at a banquet held at the Romanian embassy in Beijing on 23 August 1968, Zhou Enlai denounced the Soviet Union for "fascist politics, great power chauvinism, national egoism and social imperialism".

[27][full citation needed] By the end of 15 March Soviet General Oleg Losik ordered to deploy then-secret BM-21 "Grad" multiple rocket launchers.

[29][18] Vladimir Gorodinsky [ru] claims that the main group of Chinese troops near the island did not suffer any significant losses as a result of the usage of BM-21 due to a mistake in the calculation by the Headquarters of the Rocket Forces.

[30][full citation needed] Maxwell notes that much of academic writing about the Zhenbao incident still argues that it resulted from an unprovoked aggressive action by China and that the USSR was unquestionably the victor.

Colonel Demokrat Leonov led the group of four T-62 tanks in a counterattack on 15 March and was killed by a Chinese sniper when he was leaving a destroyed vehicle.

Senior Lieutenant Vitaly Bubenin led a relief mission of 23 soldiers from the nearby border guard's outpost and conducted a BTR-60 raid into the Chinese rear that allegedly left 248 attackers dead.

Zhou, who wanted to take up Kosygin's ceasefire offer, was shocked by what he regarded as Mao's recklessness: "The two countries are at war, one cannot chop the messenger."

Diplomats from the Soviet embassy in Beijing spent much of 22 March in vain trying to get hold of Mao's private phone number so that Kosygin could call him to discuss peace.

[35] On 22 March, Mao had a meeting with the four marshals who commanded the Chinese troops in the border regions with the Soviet Union to begin preparations for a possible all-out war.

Despite the official end of the Cultural Revolution, the Congress elected to key positions followers of the ultraleftist factions associated with Mao's powerful wife, Jiang Qing, and Defense Minister Lin Biao.

On 1 May, in a bid to repair China's image abroad, which had been badly damaged by the Cultural Revolution, Mao invited diplomats from several Third World nations to attend the May Day celebrations in Beijing.

Kim declined to move away from China, and in a show of support for Mao, North Korea sent no delegation to the world conference of communist parties that was held in Moscow in June.

[39] Ceaușescu agreed to do so, and on 7 September, Romanian Prime Minister Ion Gheorghe Maurer, who was in Hanoi to attend the funeral of Ho Chi Minh, took Zhou aside to tell him that Nixon wanted an opening to China.

After Kosygin had attended Ho's funeral, the airplane taking him back to Moscow was denied permission to use Chinese air space, which forced it to land for refuelling in Calcutta.

[7] Noting that "neither side wishes the inflamed border situation to get out of hand", the Central Intelligence Agency (CIA) in August 1969 described the conflict as having "explosive potential" in the President's Daily Briefing.

[49] The agency stated that "the potential for a war between them clearly exists", including a Soviet attack on Chinese nuclear facilities, and China "appears to view the USSR as its most immediate enemy".

Western historians believe that the events at Zhenbao Island and the subsequent border clashes in Xinjiang were caused mainly by Mao's use of Chinese local military superiority to satisfy domestic political imperatives in 1969.

[52] Yang Kuisong concludes that "the [Sino-Soviet] military clashes were primarily the result of Mao Zedong's domestic mobilization strategies, connected to his worries about the development of the Cultural Revolution.

"[53] Russian historians point out that the consequences of the conflict stem directly from a Chinese desire to take a leading role in the world and to strengthen ties with the United States.

After the conflict, the US showed interest in strengthening ties with the Chinese government by secretly sending Henry Kissinger to China for a meeting with Zhou in 1971, during the so-called Ping Pong Diplomacy.

The 9th National Congress of the Chinese Communist Party, held in the aftermath of the Zhenbao incident, confirmed Defense Minister Lin Biao as Mao's heir apparent.

On 17 October 1995, an agreement over the last 54-kilometre (34 mi) stretch of the border was reached, but the question of control over three islands in the Amur and Argun rivers was left to be settled later.

On 2 June, Foreign Minister of China, Li Zhaoxing and his Russian counterpart, Sergei Lavrov, exchanged the ratification documents from their respective governments.