Mongolia is a sandwich country between Russia and China or buffer country for Russia. When there was cooling or freezing phases in friendship between Russia and China, Mongolia or Mongols felt better. When they found common languages, not only Mongols but other nations or regions in between them suffered. For example Eastern Turkestan (today's Sintsian Uighur Autonomic Region of China). They made success in their independence movement in 1950's. But after meeting of Stalin and Mao Tse Dong, KGB organized plane crash beheading movement (leaders of Eastern Turkestan was invited for meeting in Tashkent, Uzbekistan).
And next discussion was between Stalin and Mao, concerning Mongolia.
Mao:- Comrade Stalin, we are now friends for a thousand years in building socialism and communism. So give back to China Outer Mongolia, please!
Stalin:- No, we need buffer country. Germany and Japan will raise again very soon. You'll see...