A few points to consider:

The longer the logistical past for a military forces supply train the more vulnerable and inefficient it is. Call it an inverse square law sort of thing. The US supply line to the South China Sea is substantially longer than China's. Add to that the ease of logistics by train vs. by ship across an ocean.

Japan clearly does not trust China to limit its land-grab to Taiwan. China's argument for ownership/occupation of Taiwan is well known. The Japanese fear of an invasion of Taiwan being an existential threat may turn out to be a self-fulfilling prophesy if they respond.

The "body language" signals being sent by the US administration may give both Russia, China and Israel "permission" within their spheres of influence in consideration of them letting the US do as it will in the western hemisphere (including Greenland and Canada).

Jeff