A diplomatic rift over a Japanese memorial service for Korean forced labor victims at the Sado gold mine has underscored the fragility of South Korea-Japan relations, dampening optimism in Seoul about Prime Minister Shigeru Ishiba’s leadership and threatening fledgling bilateral cooperation.

The controversy stems from a deal that the two countries reached in July, under which Seoul agreed to support Japan’s bid to have the centuries-old mine inscribed as a UNESCO World Heritage site in return for a commitment from Tokyo to acknowledge the history of Korean forced labor at the mine.

But Japan’s decision to send a senior official