The officer, Major General Leslie Groves, carried with him a bundle of photos that may have shocked even those war-hardened men.
In fine resolution, the black-and-white pictures depicted the two mushroom clouds that defined the course of modern history, and showed what was left of the Japanese cities after the mega-blasts of August 6 and 9, 1945.
Groves had directed the Manhattan Project, in which physicists including Robert Oppenheimer designed and created the world's first nuclear bombs.
"There's no defense against this bomb."
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The institution has had the photographs since the 1990s, but Krepon last year decided to gift them to the Hiroshima Peace Memorial Museum.
After negotiations about how the photos would be displayed, they will be sent to Japan in the coming weeks.
"Not many people get to see this at the Stimson Center, it truly belongs I think with them as part of the historical record, so I reached out," Krepon said of the collection of 20 or so images.
On May 27, President Barack Obama will become the first sitting US president to visit Hiroshima, where he will pay his respects at Hiroshima's Peace Memorial Park accompanied by Japanese Prime Minister Shinzo Abe.
US officials have stressed there would be no apology for the city's devastation. To this day, historians and politicians debate whether the horrendous civilian toll justified hastening the end of World War II.
About 140,000 people died after a B-29 bomber dropped its payload, code named Little Boy, on Hiroshima.