Indeed. At worst they "condemn" the actions of the US, call a session at the UN, and maybe maybe file sanctions against the US. (considering we'll be liable for any and all economic and social losses)Japan gonna move on us with the military they don't even have?
Both of those nations have tens of thousands of US troops sitting in their borders right now.
I'm not saying that our allies would "like" what we do. But there is NO scenario where South Korea or Japan join a war on North Korea's side.
It's like saying that Saudi Arabia and Israel were on the verge of joining forces with Saddam Hussain were the US invaded, cause they're all Arabish, right?