Actually I think it is his/our decision by virtue of the end of both wars
Actually, the Korean War was never ended. Hostilities were suspended; they’re technically still at war.
As for Japan, they are a sovereign nation bound by only by their own Constitution, which they are modifying to allow for offense, not just defense. Like others, I’d be disappointed if Tokyo doesn’t already have nukes.
That was changed long time ago when we gave up that right & they took back their right’s of self determination.
They are they’re OWN sovereign countries now & they shape their own future/s.
And yes, we their closest ally, but they have their leaders have their own countries to answer to, & protect.