Rintaro Nishimura is a Tokyo-based associate with The Asia Group, a strategic advisory firm. Jinwan Park is a nonresident James A. Kelly Korea Fellow at Pacific Forum.

Just three months ago, Japan and South Korea seemed to be making steady progress in mending ties, seeking to overcome the historical tensions (stemming from Japanese colonial rule over the Korean peninsula) that had long challenged the relationship. And to the delight of the U.S., ties between the three countries were also blossoming.