The ROC holds that Tibet has been part of China since 1792 at latest, which is the consensus of non-PRC historians everywhere. The Qing Dynasty fully integrated Tibet, though Tibet and China had been co-ruled as part of the same country with no meaningful distinction since the 14th century.
Are you fucking for real? A quick google search, even on the heavily CIA edited Wikipedia, would tell you Tibet ( which was part of the Mongolic Dzungarian empire at the time) was annexed by Qing Empire in the 1700s.