Hiroshima was attacked on the morning (Japanese time) of August 6th. The attack destroyed communication lines from the city so for awhile nothing was known about what happened to it.
By the evening the Japanese high command had gotten reports that it had been attacked by “a new weapon of unprecedented destructiveness.”
It was even reported that it was one bomb that did it, which brought up atomic bombs to the mind of several of the army staffers who heard about it and knew about Japan’s own work in this area.
Just before dawn (around 4 am) the US began broadcasting Truman’s statement about the bomb on shortwave radio stations in the Pacific theatre.
The Japanese high command (including the Emperor) were told about this on the morning of August 7th.
That afternoon they had a cabinet meeting to discuss the situation. The head of the Army (Anami) cast doubt on the authenticity of the claim but said they had sent a team of scientists to investigate Hiroshima.
He said they ought to wait to hear back from them until they made any decisions. This was agreed upon as a sensible thing to do.
They also agreed that they ought to start registering a protest with the International Red Cross for US violations of international law against the use of poison gas.
In his diary, though, Anami more or less made clear he believed it was an atomic bomb from the beginning, and other members of the cabinet (e.g. the foreign minister, Togo), also appeared to be acting under the assumption that it was a real weapon.
The team that went to Hiroshima was led by Dr. Yoshio Nishina, who had been a leader on the Japanese fission research program.
He and a general arrived at Hiroshima on the morning of August 8th and began examining both the characteristics of the damage (e.g., by examining knocked-down grass and trees, he could discern from what direction a uniform blast wave had traveled, for example) and the human remains (many of which showed signs of immediate high-temperature burns, and were measurably radioactive).
From that he concluded very quickly that the weapon was an atomic bomb. On the evening of August 8th sent back to Tokyo the message:
What I’ve seen so far is unspeakable. Tens of thousands dead. Bodies piled up everywhere. Sick, wounded, naked people wandering around in a daze… Almost no buildings left standing. It’s all true then? Hiroshima is completely wiped out? Completely. … I’m very sorry to tell you this… the so-called new-type bomb is actually an atomic bomb.
That night, the Soviets declared war on Japan and began their invasion in Manchuria. A cabinet meeting was called for the next day (August 9th), and while they were meeting to discuss this, they got news of a second atomic bomb attack, on Nagasaki.
The key take-away for me is that three days between atomic bombs was not a lot of time to process what happened — by the time the cabinet had met with confirmation that the first attack was an atomic bomb (which was a reasonable thing to get, and was prosecuted with due speed), the second mission had already begun.
Anyway, it was not the original American schedule either, which had originally put a more reasonable week between the first and second bombs.
The first bomb was scheduled to have been dropped anytime after August 3rd; the second was scheduled originally for August 10th. Bad weather delayed the first until August 6th, and a weather forecast caused the people on Tinian to decide to push the second one up to August 9th.
Anyway, a good source on these kind of “timeline” issue is Tsuyoshi Hasegawa’s Racing the Enemy (2005), though the above account is pieced together from a few other sources as well (Hasegawa doesn’t go into any details of the Nishina trip, for example; my Nishina quote comes from Monica Braw, The Atomic Bomb Suppressed).
– restricteddata
