The dialogue was sufficiently charged to scuttle a planned exhibit at the Smithsonian Institution in Washington on the Enola Gay, the B-29 bomber used to deploy the atomic weapon over Hiroshima.

In the Kansas City area, a group of World War II veterans formed the Harry S. Truman Appreciation Society, whose members in 1995 laid a wreath at the Truman Library grave of the former president and saluted him for using “every available weapon” to end the war.

Annual ceremonies followed. In 1998, Paul Tibbets, who had piloted the Enola Gay, spoke. Those ceremonies continue today, now organized by the Harry S. Truman Chapter of the Air Force Association.

“We believe in what President Truman did,” said Pat Snyder of Overland Park, past chapter president. “We feel that his decision saved many lives.”

Though Daniel had not added his voice to this dialogue, he often has served as unofficial family spokesman. In 2010, he helped host a 60th anniversary commemoration of the Korean War in Independence.

That same year he met Masahiro Sasaki in New York.

By then Sasaki had formed a nonprofit group to promote reconciliation between the United States and Japan, and was preparing to donate one of his sister’s last paper cranes to the World Trade Center Memorial. At one moment, Masahiro’s son placed a paper crane in Daniel’s hand.

“It was the last crane Sadako had folded before she died,” Daniel said.

“He dropped it in my palm and asked if I would consider coming to Hiroshima and Nagasaki.”

Daniel flew to Tokyo in August with his wife, Polly, and their two sons, Wesley and Gates. Daniel said he financed the trip with some assistance from a member of the Truman Library Institute, the library’s nonprofit support group.

“There are those in Japan who are still very angry about the bombings,” Daniel said.

“Most people would tell me ‘We appreciate your coming.’ But some of the meetings with survivors were very emotional.”

Following the Nagasaki ceremony, a French journalist asked Daniel, again, why he had come.

“I said the trip was about reconciliation and healing,” he said. “I didn’t try to duck anything, but neither was I going apologize for my grandfather. He never did, first of all, and the country has not.

“But I can still reach out to these people.”

Daniel recently received a grant through the United States-Japan Foundation to begin work on a book that will detail the bombings and his grandfather’s rationale for them, as well as how survivors went forward afterward.

He is seeking separate funding to travel to Japan and create an archive of survivor testimony that later would be presented to the Truman Library. Partnering with Daniel on this project is Ari Beser, a grandson of Jacob Beser, a crew member on both the Hiroshima and Nagasaki bombing missions.

“It is powerful that somebody from the Truman family would go out on a limb like this,” said Kathleen Sullivan, program director of Hibakusha Stories, a New York nonprofit that arranges for survivors to share testimony with students.

“Here you have the grandson of the man who made the decision, standing in front of students with survivors, talking about why we need to rid the world of nuclear weapons,” she said.

“The historical significance isn’t lost on the students.”

D. M. Giangreco, a military historian at the U.S. Army Command and General Staff College in Fort Leavenworth, believes Daniel’s efforts likely would benefit if he considered expanding the definition of “survivor.”

As detailed in Giangreco’s 2009 book, “Hell to Pay: Operation Downfall and the Invasion of Japan, 1945-1947,” estimates of the massive casualties possible during a traditional invasion of the Japanese home islands had been prepared by officials in the Franklin Roosevelt administration. Truman became president upon Roosevelt’s death in April 1945.

By that July, estimates circulating among senior Truman administration officials indicated that the number of Japanese dead could reach between 5 million and 10 million, with the possibility of anywhere from 1.7 million to 4 million American casualties. That included perhaps 400,000 to 800,000 American soldiers killed.

Those who ultimately didn’t have to invade Japan can be considered survivors, too, said Giangreco. Other Truman scholars such as Robert Ferrell and George Elsey recently have expressed similar sentiments, he added.

Daniel doesn’t dispute them.

“Over the years I have shaken the hands of dozens of American survivors of World War II, veterans who have told me ‘I wouldn’t be alive if your grandfather had not dropped that bomb,’ ” he said.

“Yet I have also held one of Sadako’s last paper cranes in my hand, as well. So I choose to honor both — those who fought and died for our country, as well as those in Japan who were just living their lives in a totalitarian state.”