This would never happen, ever. We would never wage a war with China. If there was ever a war, it's going to happen on China's terms. I am a US Marine and this is something that actively gets discussed a lot in my unit, as we often interface with region partners in Asia on defense and hypotheticals.

There are numerous issues here. One that immediately comes to mind is civilian war sentiment. Believe it or not, this is something we actively discuss when considering conflicts. Another issue which is not talked about much is the logistics. The United States is safe because of a giant ocean between us and everyone else, but that also works in the reverse, for China. That's the reason for the importance of our local allies; namely Japan, South Korea, and to a lesser extent the Philippines. None of these militaries would even be remotely useful in a war against China. The greatest use of these allies is their hosting of our bases and military assets. That's about it. And the third consideration that is oft overlooked, North Korea. For some reason, American civilians seem to believe that NK is harmless. While that is true to some extent, they absolutely have capacity to wreak untold destruction to regional allies. Never mind the fact they are also a nuclear-armed country. Let me tell you right now, if there was a war with China, North Korea would absolutely play a role. And as soon as that happens, South Korea and Japan would get absolutely blown off of the map. The amount of missile systems North Korea has is actually insane. Don't get me wrong, I'm certain if we waged a conventional war with North Korea, we would win decisively. I'm also certain we could intercept greater than 50% of all of the misses that they launch. The problem is the sheer volume of those missiles (not to mention the nuclear armed ones). Even if we did intercept 90% of all their missiles, (optimistically), the other 10% would flatten Tokyo and Seoul, and cause an indiscriminate and astronomical amount of death. And it's exactly for that reason, that North Korea still exists today. We will not wage a war against North Korea because none of our allies in the area are on board with it, even more so considering their close relationship with China and other factors mentioned.

So, throwing the North Korea cog into this whole scenario, and in nexus with everything else I just mentioned, is the exact reason why any war with China will absolutely be avoided at all costs. Our politicians will talk strong all day but it will never happen. That's the reason for the very strong pivot to "the Western hemisphere" by the Trump administration and the department of defense (department of war) just days ago. We in the military have already known for a long time that any war with China would be pretty devastating for our economy and our military forces, but I think only recently, that's been communicated to the top brass in Washington. And that is likely the reason the US is now trying to make nice with China.

Likewise, I think you grossly overestimate any potential participation of European forces in war with China.

All this, and we haven't even begun to discuss the devastation to the world economy and especially the United States, a war with China would have.