In August 1945, when U.S. General MacArthur (as the Supreme Commander for the Allied Powers) assigned the R.O.C government to receive the surrender of the Japanese commanders in Taiwan, the R.O.C. government was still in control of a large part of China's territory. But, by 1949, that government had lost control over most of China's territory to the Chinese Communists in a civil war and taken refuge in Formosa, outside of China's territory.
REFERENCE One-China Policy and Taiwan
by Y. Frank Chiang Fordham International Law Journal Vol. 28:1, December 2004