China, now under Deng Xiaoping, was starting the Chinese economic reform and opening trade with the West, in turn, growing increasingly defiant of the Soviet Union. China grew concerned about the strong Soviet influence in Vietnam, fearing that Vietnam could become a pseudo-protectorate of the Soviet Union. Vietnam's claim to be the world's third largest military power following its victory in the Vietnam War also increased Chinese apprehensions. In the Chinese view, Vietnam was pursuing a regional hegemonic policy in an attempt to control Indochina. In July 1978, the Chinese Politburo discussed possible military action against Vietnam in order to disrupt Soviet deployments and, two months later, PLA General Staff recommended punitive actions against Vietnam.^[222]

The major breakdown in the Chinese view of Vietnam occurred in November 1978.^[222] Vietnam joined the CMEA and, on 3 November, the Soviet Union and Vietnam signed a 25-year mutual defense treaty, which made Vietnam the "linchpin" in the Soviet Union's "drive to contain China"^[223] (however, the Soviet Union had shifted from open animosity towards more normalized relations with China soon after).^[224] Vietnam called for a special relationship between the three Indochinese countries, but the Khmer Rouge regime of Democratic Kampuchea rejected the idea.^[222] On 25 December 1978, Vietnam invaded Democratic Kampuchea, overrunning most of the country, deposing the Khmer Rouge, and installing Heng Samrin as the head of the new Cambodian government.^[225] The move antagonized China, which now viewed the Soviet Union as capable of encircling its southern border.^[226]

The reason cited for the attack was to support China's ally, the Khmer Rouge of Cambodia, in addition to the mistreatment of Vietnam's ethnic Chinese minority and the Vietnamese occupation of the Spratly Islands which were claimed by China. To prevent Soviet intervention on Vietnam's behalf, Deng warned Moscow the next day that China was prepared for a full-scale war against the Soviet Union; in preparation for this conflict, China put all of its troops along the Sino-Soviet border on an emergency war alert, set up a new military command in Xinjiang, and even evacuated an estimated 300,000 civilians from the Sino-Soviet border.^[227] In addition, the bulk of China's active forces (as many as one-and-a-half million troops) were stationed along China's border with the Soviet Union.^[228]

In February 1979, Chinese forces launched a surprise invasion of northern Vietnam and quickly captured several cities near the border. On 6 March of that year, China declared that the "gate to Hanoi" had been opened and that its punitive mission had been accomplished. Chinese troops then withdrew from Vietnam. However, Vietnam continued to occupy Cambodia until 1989, which means that China did not achieve its goal of dissuading Vietnam from involvement in Cambodia. But, China's operation at least successfully forced Vietnam to withdraw some units, namely the 2nd Corps, from the invasion forces of Cambodia to reinforce the defense of Hanoi.^[229] The conflict had a lasting impact on the relationship between China and Vietnam, and diplomatic relations between the two countries were not fully restored until 1991. Following the dissolution of the Soviet Union in 1991, the Sino-Vietnamese border was finalized. Although unable to deter Vietnam from ousting Pol Pot from Cambodia, China demonstrated that the Soviet Union, its Cold War communist adversary, was unable to protect its Vietnamese ally.^[230]