In international law, there are only a few ways that sovereignty changes.  The primary way is through a peace treaty.  Just because the ROC occupied the Taiwan island of Japan in 1945, didn't change the sovereignty of the island.  Until Japan signed the treaty giving up control of Taiwan, it was still part of Japan.Since Japan gave up control of Taiwan without naming a recipient nation, Taiwan was never reunified with China.Don't write me off as ignorant just because I don't agree with you.