We often think of nationalism purely as the dark side of in-group/out-group dynamics scaled up to the size of an entire country. They are, but there's a dark, darker, yet darker side to this. The driving force behind those dynamics is a desire to purge the in-group of perceived weakness. There are plenty of reasons for a Korean person to hate Japanese people, or vice versa, but the reason that nationalists will consistently pick is that the out-group is "inferior". War crimes, national insults, and so on are all just window-dressing to that core claim.
This same dynamic also applies to things you would not consider to be even in the same postal code as right-wing nationalism. Think about how much self-policing and internal policing happens in niche subcultures online to purge them of perceived weakness, and how many of the perpetrators of those purges are not much different from their targets. Nationalists care more about punishing their own citizens than fighting the enemy.
Long story short, Japanese politicians should have known better.
On the other hand we have something similar in the USA so who am I to judge? :)
One party in our two party system appears to have lots of Russian connections now for some reason, and this does not bother most of their supporters which has approximately 45-50 percent of the population behind it. In past years this would have led to claims about treason or something similar from the same party, the path from Nixon to now it appears the only change has been how public the cooperation between the GOP leadership and countries openly antagonistic to the USA, i.e. Nixon/Kissinger with North Vietnam, Reagan/Bush with Iran and Trump with Russia.