The motivation in reality is power for China by taking over lands, even to the point of creating them, that they have always declared to be theirs although the rest of the world said it wasn't. Kind of like Russia trying to rebuild the Soviet Union by force.
I'm failing to understand what your particular point is.....China clearly has an agenda and a point...thumbing its nose at those who oppose their actions but do nothing to stop them. I'd like to point out, although Bob will again claim 'it's in the past', who was president when China started their creation of land masses in that area and did nothing? Just like with illegal immigration.....ignoring it doesn't make the problem go away; it only gets worse, and then you have somebody else that has to deal with those consequences.
The article says 'US could kick China's butt almost anywhere in the world'.
https://www.reuters.com/article/us-apps-china-commentary/commentary-how-beijing-is-winning-in-the-south-china-sea-idUSKBN1H42JY
The rest of it is not as comforting to you.
Warning. Long read.
BTW as in past changes of empire, the motivation is economic. Oil and fishing.

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