But also a lot of uncertainty whether the U.S. will be there for them going forward. Their own self-interest appears to forcing them to work together more closely than before.
Both sides can see the writing (either hangul or kanji) on the wall: regardless of the past state crimes of Japan, they both will need each other, along with Taiwan and Viet Nam at least, to stand together against China. If they can bring the Russians in without getting the bear’s nose too far into the Asian tent, so much the better. OTOH, if Trump is smart and picks someone like Petraeus for under secdef for Asia, this can get even better, as we rebuild our own intel and join with the Japanese and SKs.
President Trump’s plan for SE Asia is already working and he hasn’t even assumed office yet!