If North Korea attacks South Korea, the U.S. gets involved simply by virtue of the personnel it has based there.
U.S. in Japan will play a role in any conflict.... what if Pyongyang lobs a couple of missiles at those bases?
Here’s something of interest.
The USN is currently swapping out it’s Japan based forward deployed carrier. The former deck, the Washington, and the future deck, the Reagan, are both on the US West Coast for turnover ops. Along with their airwings and escort ships. Well out of range of a potential Nork first strike.
Think of the impact of the US carriers not being in Pearl Harbor on Dec 7, 1941 ...
That was the assumption up until January 20, 2009.
This CiC would order a stand-down and get-out.
And NK has to pick its battles: lobbing something at Japan would serve no purpose whatsoever, better (however bad that still is) to focus on “just across the border”. It’s conceivable that a valiant strike, followed by total NK collapse, would actually prompt unification of Korea - so long as NK doesn’t draw USA & Japan into the conflict directly. There is a _very_ strong unification intention simmering, don’t underestimate it - a la East/West Germany. It would play out akin to a mentally unstable member _finally_ accepting treatment after beating the crap out of a sibling/spouse (so long as neighbors & police weren’t attacked in the process).