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The nine dash line is a creation of the CCP, not of the ROC. It was not claimed from the ROC's time.That's factually incorrect. The ROC calls it the eleven dash line (PRC removed 2 dashes at the border with Vietnam) and they were the ones who created it before the PRC even existed [1]
Mainland China has taken possession because they are now strong enough to do it. It was only a claim before because they were too weak to assert their claim physically, which any large power would have done, or to make the others recognize that claim.
> other countries' already existing territory
The borders of all the countries in the areas were decided by Western colonial powers, and China was simply too weak to assert its interests and wasn't invited.
There is not right or wrong here. This is simply power and how things have always worked. It's naive to buy the "good guys bad guys" or "bully" narrative, which is only propaganda from one side against the other.
[1] https://en.wikipedia.org/wiki/Nine-dash_line