@histoire @alyx @VD15 3 years ago or so, I watched a presentation and I talked to an ambassador from Japan.
It basicly comes down to Japan is not allowed to have a offensive military force, and they are basicly relying on the US to protect them. So with North Korea, they were really worried about them having missiles that could attack them directly. Also with Korea and Russia being so close to them, they are worried about what they will do.
I am not 100% sure, but Trump has had a relatively good relationship to North Korea and Russia, while pushing back against China.
With what Obama was like, they might be worried about Biden doing the same thing.