[natded]

The Taiwanese leader is hardly "standing up" to China. China's calculus of attacking Taiwan is probably a more global in scale: for example US submarines can end the maritime traffic in South China Sea completely at any given point.

[seanmcdirmid]

SCS is not conducive to submarine operations, which is why China wants to monopolize it so badly. Otherwise, they are completely hemmed in by Korea, Russia, Japan, Taiwan. Unfettered sea access for trade or military reasons is seen as very important, right now it is easy to cut China off (and Russia is still considered a frenemy despite how relations have improved in the last 20 years).