US will resort to anything within its might (economical,
political power) to prevent a war between South Korea and Japan. In
the event that such a war does eventually break out and US fails to
prevent or stop it, that alone signals one thing: the declining of
US power and influence in this part of the world. But I don't think
we've reached that moment (full scale US decline) in time. It'll
happen, but not so soon.
Besides, in times of war involving South Korea, US has ABSOLUTE
command of South Korea's military. Absolute command, over each and
everyone of the South Korean soldier and officers. It's a South
Korean law.
So, I don't think a war will break out. At most some low scale
skirmishes.