Let's deal with these issues one at a time.

China has claimed the whole of the South China Sea as it's own. Many oof the areas claim are inside the territorial waters of other Sovereign states. It has been reclaiming land around disputed islands and building port facilities and airfield there. That alone would be enough to start a war any where else in the world. China is flexing it's muscles to test US resolve. So the US sends out air patrols, increases the concentration of surface vessels in the US fleet and refuses to be intimidated or provoked.
Types of warfare
Conventional
Theatre Strategic (Short range nuclear)
Nuclear

As this link demonstrates, the US beats the Chinese into a cocked hat on any level of conventional or strategic warfare capability. The US has recent conventional warfare experience and highly trained well equipped troops. On numbers alone it would not be difficult to face down the Chinese, but the US also has better equipment and recent experience of using it in combat.

The first phase of warfare is to establish air superiority. Have a look at the comparative strengths and tell us how you think the Chinese would stack up against the US.

Neither side wants war because war is expensive and dangerous. The last thing either side wants is attrition of manpower and equipment. Most of it is best left to diplomacy. Today te US has dominance in almost any military scenario so it can afford to use diplomacy backed by a very big stick. Next year or more likely 10 years from now, China may be catching up.

I won't mention the Aussies or the Japanese, but you should not count against the goodwill of other US allies in the region. South Korea and the Philippines will reciprocate with US protection of their rights against a common enemy if necessary.