Also I'm sure that Japan will throw their hat in the ring for the US, I'm not quite sure on SK since it seems almost suicidal to me. Both assuming that the JSDF and the ROK armed forces join in, wouldn't their missiles also add to the US side of the equation and result in more disabled targets even considering the degraded capabilities of TLAM's directly after the intial salvos?
Japan and ROK don't have that many missiles currently.
As a side note, it seems to me that if Japan is as in lockstep with the US as they seem to be, the best strategy would be shift the focus of the JSDF and make something equivalent to the PLARF, since I assume Japanese Naval assets will probably a nonfactor and Japanese airbases will also not survive. What do you think the US should have Japan and SK do in order to tip the scales more in their favor? Do you believe that there is political will for Japan (probably imo) and SK (probably not imo) to stomach the type of war that you envision?
What you're suggesting is that Japan becomes an offensive missile appendage for the US military.
Japan would need constitutional amendments to allow for an official military to exist and also renounce the anti-war clauses. Then significant budget increases would be needed to pay for new missiles, which we might see arriving in 5 years time. But within the next 5 years , we'll likely see the 300+ ICBM silos in China comprise a sufficient deterrent against any country declaring war (and therefore a pre-emptive first strike) against China.
But let's say that Japan does militarise and field large numbers of land-based missiles.
What would be the Chinese response?
We see China running a lot of military training exercises in the East China Sea in peacetime, with the Japanese Navy, Coast Guard and Air Force exhausting themselves in trying to monitor this. Yet the Chinese military is much larger and still has huge scope to increase their operating tempo. Furthermore, overall Chinese military spending is still growing in tandem with the economy, so we can expect even more Chinese training exercises in the future anyway. That forces Japan to spend more on the Navy, Coast Guard and Air Force if Japan wants to assert sovereignty over the disputed Diaoyu/Senkaku Islands, and is a competition that Japan can't win if China chooses to up the operating tempo. It also means the Japanese military has to get used to presence of Chinese military forces in the East China Seas as a matter of routine.
China's GDP is currently 6x larger than Japan, and China adds a Japan-sized economy roughly every 4 years. In a missile arms race, China would likely build even more offensive missiles than Japan, so that Japan comes off far worse off in any Japan-China conflict. It also raises the possibility of nuclear escalation and Japan doesn't have any nukes. The other thing to remember is that the Japanese Home Islands are geographically small and located on the fringes of Asia. Japan has to import almost all its natural resources and pay for these with manufactured goods, which will be difficult if it is at war with China. So Japan needs to maintain a significant Navy to escort its merchant shipping. In comparison, China is the world's largest trading nation and also sits in the middle of the Asian (and Global) trading network. China is also geographically the same size as a continent-spanning USA, so China could be largely self-sufficient just from the resources within its own borders. Plus China can rely on overland trade for many needs.
At the same time, look at the previous economic responses to THAAD deployments in Korea or when Japan arrested Chinese fisherman in the disputed islands.
Or China could be a lot more subtle. For example, China accounts for roughly 75% of the entire electric vehicle supply chain. If Japan is blacklisted or impeded from accessing the Chinese electric vehicle supply chain, Japan won't be able to leverage the lower costs and better technology available in China. Firstly, we would see Japanese carmakers displaced even faster by Tesla and the Chinese carmakers in China, then in overseas markets and then inside Japan to a greater degree than otherwise.
In Japan, approximately 20% of manufacturing employment is in the automobile industry and it accounts for $80 Billion of exports from Japan each year. In 2021, approximately half of all cars made in Japan were exported. So if Japan loses its car exports, the domestic Japanese auto industry should halve in size. Then if we start seeing significant imports of Tesla and/or Chinese electric cars, the domestic Japanese auto industry would shrink even further.
A similar analysis applies to many other industries in Japan and also to other countries in Asia and the rest of the world.
So if the China-Japan relationship gets worse, there potentially would be less tax revenue available to pay for an arms buildup against China. It's a Catch-22 for Japan.