Anyone who reads unbiased history will know that Yingwai Suchaonovich’s assertion that Tibet has been part of China since the Mongols conquered both countries is simply false.
During the Ming Dynasty, neither Tibet nor China regarded Tibet as part of the imperial domains. A special bureau within the Ming government was devoted to relations with Tibet, and there was no question of either Tibet or Mongolia being a Chinese province.
Even during the Ch’ing period, the Manchus did not exert any direct control over Tibet until the middle of the 18th century. When the Ch’ing fell, both Mongolia and Tibet reasserted their independence, which lasted until the communist take-over.
By the way, no one except extreme Chinese chauvinists considers Mongolia a part of China.
Please read any of the scholarly histories available in many languages, and China’s claim will be seen to be a lie.