history of the nine-dash line is super interesting - essentially a US cartographic invention, dividing up the maritime areas previously taken over by Imperial Japan. China - then under control (more or less) of the nationalists - were a classic US protege regime and given the bulk of the territory, as the safest bet amongst all other claimants, all of whom were undergoing nationalist insurgencies against the restoration of European colonial control. The US did not anticipate, I guess, that the CPC would defeat the Nationalists and drive them to the island of Taiwan, and so here we are