The US failure in the Vietnam War wiped out the notion that communism had to be contained at all costs. Therefore, the US came to interpret China in a different way. Before the Vietnam War, as Jian Chen describes it, the US had generally regarded China as an “aggressive, violent, and irrational” communist country. The US had not paid as much attention to Chinese nationalism as it did to the Sino-Soviet ideological relationship: China was nothing but a Soviet ally. But as the US got bogged down in the Vietnam War, it realized that Asian nationalism played a much more important role than it had imagined. As a result, American foreign policy toward Asia became more sensitive than before to Asian nationalism. In particular, the US turned its eyes to the historical boundary dispute between the Soviet Union and China.
Up until the end of the 1950s, relations between the two communist countries had seemed close due to their expanding trade. But relations gradually deteriorated in the 1960s. On the surface, the deterioration came from an ideological disagreement over the interpretation of Marxism. However, the real reason derived from the historical rivalry and mutual fear of the two former empires.
For the Soviet Union, Communist China was a myth from its birth since it had assumed that the Nationalists would reassert control over China in the wake of the Japanese surrender. After China became a communist country in 1949, Soviet leaders still felt China was different from its other European satellites (except Yugoslavia). Joseph Stalin felt Mao was not very loyal to the Soviet Union.
On the other hand, Mao himself became very cautious about Stalin after having realized Stalin’s cold attitude in providing promised military aid to the Chinese in the Korean War. Moreover, Mao disliked the Soviet imperialistic policy and felt that the Soviet Union would never understand Chinese nationalism. Both Mao and the Soviet leaders were extremely nationalistic in terms of their vast territories, and they did not really trust each other. Thus the Sino-Soviet dispute went beyond ideology.
As a result of this dispute, the Soviet Union first refused to assist the Chinese in developing nuclear power and then withdrew its economic assistance and technical advice from China in 1960. China, in turn, demanded that the Soviet Union return territories that had been considered Chinese in the 19th century. The Chinese also accused Soviet leaders of imperialism after the Soviet-led invasion of Czechoslovakia in 1968. And in 1969, the Chinese attacked Soviet border guards in Manchuria, making Sino-Soviet conflict inevitable.
This time, the Soviets lost China; it was a Soviet failure in East Asia.
Like the Americans in the 1950s and 1960s, the Soviets had not realized the importance of Asian nationalism. Nationalism was proving mightier than communism. This weakened position of the Soviet Union and the American failure in the Vietnam War combined to bring about a policy of détente in the 1970s.
Détente was characterized by Soviet-US cooperation on environmental protection, science and technology, space ventures, and arms limitation. Nevertheless, détente did not last long. In December 1979, the Soviet Union invaded Afghanistan to save a Marxist regime. In the end, the Soviet Union had not learned a lesson from its failure in Asia because this invasion cost the Soviet Union dearly. Amid condemnation from the rest of the world, it expended much energy and time fighting a nationalist resistance. Finally, it was obliged to withdraw all Soviet troops from Afghanistan by February 1989, acknowledging that the intervention in Afghanistan had been a mistake.
Meanwhile, Cambodia and its nationalism fell prey to the power game. In 1970, the US had masterminded a coup against King Sihanouk, in which American-supported General Lon Nol seized power and sent his army to fight the Vietcong in the border areas between Vietnam and Cambodia. The Khmer Rouge, Communist party guerrillas, aided by North Vietnam and Sihanouk, resisted Lon Nol, starting a guerrilla war.
This liberation movement expanded in the early 1970s. In April 1975, the Khmer Rouge seized Phnom Penh, forcing Lon Nol to flee the country. However, since the Khmer Rouge headed by Pol Pot soon became notorious for massacring an estimated two million people, the now reunited Vietnam intervened in Cambodia in 1978, resulting in a Cambodian civil war between the Vietnamese-backed regime led by Heng Samrin, and later by Hun Sen, and the Khmer Rouge remnants.