On Aug. 6, 1945, Maj. Gen. Leslie Groves sent a top secret cable to his superiors in Washington, D.C.

In the now declassified cable, Groves, who was in charge of the atomic bombing of Hiroshima, described what had happened.

“First there was a ball of fire, changing in a few seconds to purple clouds and flames boiling and swirling upward,” he wrote. “Entire city except outmost ends of dock areas was covered with a dark grey dust layer which joined the cloud column.”

The results, Groves told Washington, were “clearcut, successful in all respects.”

Groves’ memorandum, not publicly released until decades later, is just one of countless top-secret cables, meeting minutes, memorandums and decrypted Japanese messages sent in the days and weeks leading up to the decision by the U.S. to drop the world’s first atomic bomb on Hiroshima.

With U.S. President Barack Obama set to visit that city on Friday, the debate over the bomb’s necessity and morality has been rekindled on both sides of the Pacific.

The question is often rooted in long-standing media and popular culture-generated conceptions — and misconceptions — about the period and what leaders in Washington and Tokyo were thinking in the fateful weeks leading up to the bombing.

It was not until the 1960s that many primary sources about the atomic bomb decision, in the form of declassified U.S. government cables, began to become available. Today, many are stored on the National Security Archive website, offering researchers, professional and amateur, a trove of official documentation about the decision.

On April 27, 1945, as Germany was about to surrender, ending the war in Europe, U.S. military brass and nuclear scientists met in Washington for the first time to discuss the atomic bombing of Japan. Though the weapon was still under development, the meeting’s purpose was to discuss how, when and especially where it should first be dropped.

In a top-secret memo of the meeting, eight possible targets — Tokyo, Yokohama, Nagoya, Osaka, Kyoto, Kobe, Yawata (a steel works area in Kitakyushu) and Nagasaki — were listed, and four — Hiroshima, Yawata, Yokohama and Tokyo — were commented upon.

“Hiroshima is the largest untouched target on the 21st Bomber Command priority list. Consideration should be given to this city. Yawata is an area that should be considered … and is on the A priority list (steel industry). Yokohama is lower on the priority list of targets,” the memo said.

“Tokyo is a possibility but it is now practically all bombed and burned out, and is practically all rubble with only the palace grounds left standing,” it said, referring to the March 1945 firebombing of Tokyo that killed more than 100,000 people.

By June of that year, the Battle of Okinawa was ending. U.S. leaders were looking ahead to the next step: an invasion of Japan itself.

In a June 18, 1945, meeting, U.S. President Harry Truman and the Joint Chiefs of Staff prepared to dispatch over 766,000 troops for an invasion of Kyushu, to commence on Nov. 1.

The meeting minutes read: “It seems that if the Japanese are ever willing to capitulate short of complete military defeat in the field they will do it when faced by the completely hopeless prospect occasioned by (1) destruction already wrought by air bombardment and sea blockade, coupled with (2) a landing on Japan indicating the firmness of our resolution, and also perhaps coupled with (3) the entry or threat of entry of Russia in the war.”

Among the items discussed that day was whether the U.S. should demand Japan’s unconditional surrender. Adm. William Leahy was against it, saying he feared an insistence on unconditional surrender “would result only in making the Japanese desperate and thereby increase our casualty lists.”

No decision was made.

But by the end of July, the testing of the atomic bomb in New Mexico was a success and unconditional surrender had become a firm condition. While Hiroshima remained on the target list, the ancient city of Kyoto was removed, reportedly at the insistence of Secretary of War Henry Stimson, who had honeymooned there.

Meanwhile, U.S. intelligence was intercepting messages from the Japanese Foreign Ministry, which had approached the Soviet Union, seeking help to end the war.

Japan appeared to be unaware that the Soviets had already decided to declare war once a neutrality pact expired the following month.

“Tokyo again says no to unconditional surrender; Sato pleads for peace,” said a top-secret cable on July 22, summarizing discussions between the Soviet Union and Japan on July 19.

“With regard to unconditional surrender … we are unable to consent to it under any circumstances whatever (sic),” Foreign Minister Shigenori Togo was quoted in the decrypted cable memo as telling Ambassador Naotake Sato in Moscow.

Preparations for dropping the bomb accelerated. On July 24, a secret U.S. War Department cable said the Army Air Forces “will deliver the first special bomb as soon as the weather will permit visual bombing after about 3 August 1945 on one of the targets: Hiroshima, Kokura (in Fukuoka Prefecture), Niigata and Nagasaki.”

Just under two weeks later on Aug. 6, the decision was made. A top-secret memorandum Groves sent after the bombing stipulated the reason for the choice of location:

“The target used was Hiroshima, the one reserved target where there was no indication of any POW camp.”

The atomic bomb was then dropped.

Negotiations to end the war would continue, with the Soviet Union formally entering the conflict on Aug. 9, the same day the second atomic bomb was dropped on Nagasaki.

Last year, a diary from a senior U.S. official in 1945 was published. In it, the official said that a few months after the war ended, Gen. Dwight D. Eisenhower, who would later become president, said privately he had hoped the war would have ended without the use of nuclear weapons.

But on Aug. 13, 1945, with no announcement of surrender from Tokyo, a memo of a phone conversation between Col. L.E. Seaman, an associate of Groves, and Gen. John E. Hull, assistant chief of staff for the War Department’s Operations Division, made it clear more atomic bombs were being prepared for the coming weeks.

“You have a possibility of seven, with a good chance of using them prior to the 31st of October,” Seaman said.

Two days later, however, on Aug. 15, Japan surrendered unconditionally.

The war was over.

But as history would show, the debate on the use of atomic weapons in Hiroshima and Nagasaki was just beginning.

This is part of a series of articles spotlighting the historic visit by U.S. President Barack Obama to the atomic-bombed city of Hiroshima this week.

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