Depending on what they get from China in return, it might not be as absurd as you think. We're not talking direct participation here after all - China has more than enough people. Just some "lethal aid" if e.g. artillery stocks start running low. Another clever and relatively cheap way to extend us would be to stir up trouble where our troops are stationed in the Middle East, for example. This trouble could also use the "lethal aid" from third parties, who would not be directly involved in any fighting.
BRICS will support China. Most of the unaligned countries will do the same.
It will be US, EU, Canada, Australia, Japan, Philippines against the rest of the world.
And it's not so much about military might as is about industrial capacity, raw materials and logistics.