Post WW2
For Taiwan, following the Allied powers' directive, Japanese forces in Taiwan surrendered to Chiang Kai-shek's Republic of China (ROC) government, enabling the ROC to assume administrative control and establish its governance on the island.
The newly appointed PM made strong remarks, stating that a Chinese attack on Taiwan could be considered a "survival-threatening situation" for Japan. This prompted a response from China, including formal protests and accusations of interference in its internal affairs.
For Ryukyu(Okinawa), The U.S. occupied Okinawa until 1972, when administrative control was transferred or returned to Japan. At the UN, Chinese officials have made statements that some perceive as challenging Japan's sovereignty as a geopolitical tactic, though not as formal claims of territory.