The reason Japan and South Korea will work with the US is because China is "eating their lunch," so to speak.
Advanced East Asian economies compete over the same industries - high tech., advanced manufacturing, ship building, IT, media exports, etc. China is beating Japan - and increasingly South Korea - in all of these domains. They're losing market share and orders. Their economic life lines are more and more dependent on supplying components and tools to Chinese companies, and with the shift to self-sufficiency under Xi Jinping due to national security concerns, even that line is being cut.
This is why they will fall in line behind the US - because it's in their interest to do so. China is beating them, and they have no way to address that but to knock China down, because they have neither the market size nor the capital to compete with China directly. They need the US to sabotage China, because otherwise they're toast.
Now, is there an alternative to what they're doing? Sure. They could reach an agreement with China over establishing a collaborative supply chain, where China agrees to depend on them for certain products and components, and vice versa, so they'll always have industries that they "own" and which China largely stays out of. But to do that, they'd have to abandon their alliances with the West, first, otherwise they can never be reliable components in a Chinese supply chain.
That, however, they will never do. You won't ever convince the Korean or Japanese public that they should rather be friends with China, than the West. This is because their societies are deeply infiltrated by Western media and intelligence. They have already convinced themselves of "China bad."
So they will march to the drum of inevitable war and be the front line soldiers of future Western aggression.
