egotism. The conflict was now out of control and was increasingly fought out in 81 communist parties around the world. The final split came in July 1963, after 50,000 refugees escaped from Sinkiang, in Western China, to Soviet territory to escape persecution. China ridiculed the Soviet incompetence in the Cuban Missile Crisis of 1962 as adventurism and capitulationism that ended up in defeat. Moscow was increasingly prioritizing friendly relationships and test ban treaties with the United States and the United Kingdom. Increasingly, China began to consider the Soviet Union, which it viewed as social imperialist, as the greatest threat it faced, more so than even the leading capitalist power, the United States. In turn, overtures were made between the PRC and the US, such as in the Ping Pong Diplomacy and the 1972 Nixon visit to China. From 1965 to 1988, the Sino–Soviet border, including the Tumen River area, became highly militarized and fortified. That included a large concentration of tactical nuclear-armed missile sites on both sides of the zone. Until 1991, foreigners, consulates and nonresidents had not been permitted in Vladivostok since 1948 or in Yanbian or the border areas of Heilongjiang Province since 1965. Political, social, and economic conditions deteriorated further, as the Cultural Revolution disrupted Chinese life and institutions from 1966 to 1972. Periods of extreme tension in 1968 to 1970 along the eastern Sino–Soviet border (with Primorsky) resulted in border skirmishes on the Ussuri River in 1969 and again from 1979 to 1980, when Vietnam invaded Cambodia, and China retaliated by launching a border war with Vietnam. The skirmishes led to the intensification of border fortifications and the mobilization of the civilian populations on both sides. Post-Mao era and stabilizing relations In September 1976, Mao died. A month later, the Gang of Four were overthrown by his successor, Chairman Hua Guofeng, with the support of Deng Xiaoping, who was to soon implement pro-market economic reform. With the PRC no longer espousing the anti-revisionist notion of the antagonistic contradiction between classes, relations between the two countries became gradually normalized. In 1979, however, the PRC launched the Sino–Vietnamese War. The invasion of Vietnam, a Soviet ally, was in response to Vietnam's invasion and subsequent occupation of Cambodia, which overthrew the Chinese-backed Khmer Rouge from power. Even though Soviet leader Mikhail Gorbachev went on to criticize the post-Maoist CCP for allowing PRC millionaires to have lost the socialist path,