American foreign policy has never been to "die for x country" other than existential conflicts like WW2. It doesn't matter how much americans love one country or the other, as much as the strategic reasons behind the war. This is what we have seen in the previous decades of conflicts that the US has been involved in.

Well, South Korea and Japan being involved would just be one factor that makes justification even easier. The real reason would be national and global security. It's in the interest of preservation of freedom. You don't wait until the enemy is at your doorstep, because that means you allowed them to snowball an avalanche at you. You meet them at their doorstep before they've gained full momentum.

Could you please consider making your points without "you'd have to be insane/an idiot to believe X"?