HAPCHEON, South Korea — When President Obama visits Hiroshima this week, a small delegation from South Korea plans to gather around an obscure monument there dedicated to a group of victims who endured more than their share of misery, yet whom few remember.

As many as 220,000 people were killed by the atomic bombs that the United States dropped on Japan, most of them Japanese civilians. But 40,000 to 50,000 of the dead were Koreans who had been taken to Hiroshima or Nagasaki against their will as forced laborers, or had settled in the cities after fleeing deprivation in their occupied homeland.

Those who survived and returned to South Korea after the war were then shunned and denied medical care, partly to avoid upsetting the official view that the nuclear attacks were necessary to liberate Korea. Some were banished to leper colonies.

In the debates prompted by Mr. Obama’s coming visit, few have as many claims to an apology — from Japan, from the United States, even from their own government in South Korea — as these Korean survivors. Their unique experience illustrates the complicated, emotional politics of memory and morality that Mr. Obama must navigate when he becomes the first sitting American president to visit Hiroshima on Friday.