 Complete the following phrase:  From 1965 to 1988, the SinoSoviet 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 SinoSoviet 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 