China's invasion and occupation of Tibet in the 1950s was justified by a claim that Tibet already was part of China. The government of China said that Tibet had been part of China for nearly 700 years, and that Tibet was only temporarily separated from China for a time during the 19th century by the machinations of Western powers.
The truth is much more complicated, and also much less on China's side. The About.com Guide to Asian History, Kallie Szczepanski, has written a history of China and Tibet that I urge everyone to read. She discusses the bases of China's claims and shows them to be tenuous. The historical relationship between China and Tibet is complex, and the boundaries and balance of power between China and Tibet have shifted over the centuries.