It’s a little more complicated than that. After Japan was defeated, every country started fighting over territory in the South China Sea. The nine dash line was something the Republic of China came up with in 1946, before the Communist revolution even happened. In fact, it used to be called the eleven dash line before the PRC decided to stop fighting with Vietnam over naval borders. Taiwan still claims they are the rightful controllers of the eleven dash line. It’s not stealing so much as it’s not giving up what they already have.