Excerpt from page 6
.... after Japan's defeat in 1945, Taiwan and the Pescadores were assigned to the Republic of China for purposes of post-war occupation. Taiwan was still under this occupation four years later, when the ROC government fled to Taiwan after the communist victory in the civil war on mainland China.
Sino-Japanese Relations: Issues for U.S. Policy
Congressional Research Service
December 19, 2008
CRS Report for Congress