How has mongolia been separate from china in modern history

The bilateral relations between Mongolia and the People's Republic of China have long been determined by the relations between China and the Soviet Union, Mongolia's other neighbour and main ally until 1990. With the rapprochement between the USSR and China in the late 1980s, Sino-Mongolian relations also began to improve. Since the 1990s, China has become Mongolia's biggest trading partner, and a number of Chinese businesses are operating in Mongolia.In December 2016, China blocked its border with Mongolia in protest after Mongolia allowed the Dalai Lama to visit the country despite demands from the Chinese to cancel the visit in November 2016

This movement is also referred to as  glacial plucking