In the mid-1960s the Soviet Union began to station massive troops in the People's Republic of Mongolia and along the border between the Soviet Union and China. At the end of the 1970s it supported Vietnam in the invasion of Kampuchea and then sent troops to invade Afghanistan. These moves posed a grave threat to China's security and became the three major obstacles to the normalization of relations between China and the USSR. Between 1982 and 1988 special envoys from the governments of the two countries held consultations on the normalization of relations. The Chinese side noted that before normalization could be achieved, the Soviet Union must remove the three major obstacles. In other words, it must withdraw its troops from Mongolia and the border area, withdraw its troops from Afghanistan and urge Vietnam to stop its aggression against Kampuchea and to withdraw its troops from that country.