Its obviously the American (or should I say Israel's) intent to use Japan as fodder and, in their minds at least weaken China first in a proxy war, provide indirect support without exposure and only jump in after Japan is spent. But you can also look at this another way:
- Chinese calculus always assumed fighting US and Japan at the same time, theres no scenerio where US and China gets into a direct conflict with and Japan gets to stay unused.
- If US thought they can take on China they would be willing to jump in on day one along with Japan, so to send Japan up first is a tacit admission that they have no confidence even fighting along side Japan and is afraid of damage to themselves
- A full scale war with the US will require war time mobilization of society and industry which takes time.
So if US do send Japan up first instead of together, it can also be seen as breaking up the enemy into smaller easier to deal with chunks, while giving China time to mobilize in a relatively safe enviroment. It also opens the potential of US staying out altogether and China getting the oppertunity to neutralizing Japan without ever fighting the US.
IMO the main consideration for China should be to have full preparation for fight both at the same time. China need to ensure there is the strategic option of attacking American cargo ships trying to resupply Japan, and down American aircrafts providing support, but not expand to target US assets elsewhere unless they engage first. At the same time Chinese ships should be deployed far out into the Pacific, outside of Japaness threat range and in place to immediately engage all US forces if they join.
During which time China should also immediately mobilize the economy assuming there will be a full scale war with US, which, if the west provides Japan with any indirect support, include full embargo of any and all industrial goods from all Asian countries going to the west, at a level that makes RE and Nexperia embargo look tame. If the west's caculus is to weaken China before jumping in, then its counter is to make sure China actually gets stronger relative to them.
The entire proxy war concept rest on the assumption that the target do not wish to directly engage the proxy's handlers, e.g. its Russia's inability to fight NATO that allows NATO to operate their Ukraine proxy. If this assumption breaks down, then the proxy war turns into a piecemeal feeding instead. China should not attack the US unless they engage, but China should be fully prepared and demonstrate through sinking supply ships the ability and will to do so.
