As President Obama prepares to visit the Japanese city of Hiroshima on Friday, the debate about whether the United States should apologize for dropping an atomic bomb on the city in 1945 has been reignited (even if the White House has said that Obama has no plans to apologize).

This subject matter is perhaps not quite as straightforward as you might think. Although more and more Americans seem to question the morality of dropping the bomb, many in Japan say an apology isn't required. Meanwhile, one of the strongest voices calling for Obama to apologize comes not from Japan — but from South Korea, a country that has long been at odds with Japan over the legacy of World War II.

In a letter sent to Obama recently, a group called the Association of Korean Atomic Bomb Victims urged the U.S. president to "offer an apology" when he visits Hiroshima. The letter also accused the United States of evading its "responsibility" for the "inhumane" wartime act, Japan's Kyodo News reports.

Given South Korea's long-standing anger over Japan's wartime actions, the letter may seem surprising. However, it is a reminder that the bomb dropped on Hiroshima didn't kill only Japanese citizens. It killed Allied prisoners of war (including at least 12 U.S. soldiers) and small numbers of Chinese and Southeast Asian nationals. And those figures are dwarfed by the number of Koreans thought to have been killed in the bombing: 20,000 or more, according to most estimates, with thousands more killed in the subsequent Nagasaki bombing.

Those Koreans had mostly been working in Hiroshima at the time of the bombing, with many conscripted by force by Imperial Japan, which at that point was occupying the Korean Peninsula. One monument to the Korean victims at the Hiroshima Peace Memorial Park says that about 10 percent of those who died in the bombing had Korean lineage.

While the Korean Hiroshima victims don't have as high an international profile as the "comfort women" at the center of a decades-long dispute between Seoul and Tokyo, their case is similarly bitter and complex. Japan provided unlimited medical aid and other benefits to Japanese citizens who survived the bombings. However, Korean victims say the limited assistance they were given is not enough. Tokyo contends that a 1965 treaty to normalize ties with South Korea closed the case on any claims from victims, but this has not stopped new claims from being made.

The United States, too, has been singled out for not providing support. "Only 6 percent of the bombing victims, or 2,584, are currently alive and they have not received any compensation or apology from the United States," a shelter in South Korea's Hapcheon county that supports Korean victims of the bombing said in a statement this year.

Many of the Koreans who survived the bombing now reside in Hapcheon, a rural county, which has been nicknamed "South Korea's Hiroshima." On Saturday, about 300 people gathered for a picnic there to honor those who died and those who suffered during the Hiroshima bombing. Many were well aware that Obama would soon visit the Japanese city. “My father was living in Japan, and he died when the atomic bomb was dropped,” Yoon Il-nam, an 84-year-old woman, told Korean newspaper the Hankyoreh. “It won’t make any difference now, but we should receive the apology that we’re due. The victims are here in Hapcheon, so I have no idea why the American president is going to Hiroshima instead of coming here.”

The White House, lobbied by South Korea before Obama's trip, has acknowledged the suffering of the Korean people because of the Hiroshima bombing. "The president is conducting this visit in large part to honor the memory of all of those lost during the war and those lost in the bombing," Daniel Kritenbrink, senior Asian affairs director at the National Security Council, told Yonhap News, adding that there were "many thousands of Koreans who died in the bombing."

However, the White House has not said whether Obama would visit the monument to Korean victims at the Hiroshima Peace Memorial Park on Friday.

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