Okay, I think I'm understanding you now Flum, in everything I've found in studying the mentalities of both sides during that time I find it unlikely that the Japanese people would have surrendered at the time of the war without some type of shock factor involved. The carpet bombs didn't seem to spur that effect and the suicides of even women rather then being taken captive was rather shocking. But you are right, trying to enforce a barricade of the island without engaging in other military engagements might just have ended the war without the use of nukes. The question still remains though, what would a non nuclear ending have done to the state of the world? Specifically with what a fragile state the world was in at the time, a drawn out blockade could have been a disaster and left the war lingering. The other unknown is, had the nuke not been used, would war with the USSR have ensued without the deterrent of nuclear war being in place? Questions I can't answer, but they put question to the claim that hindsight is 20/10
As for the use of 2 close together, your point is well taken. The shock factor of the weapon itself did not need more then 1 demonstration, especially with the location of the first target. So yes, it does seem likely that 1 would have been enough.
The threat on the POW's was part of the mindset of some of Japan's military leaders who ordered kamikaze attacks. After the nuclear attacks the Emperor, in a nearly unprecedented act, got involved and spoke out saying:
"I have given serious thought to the situation prevailing at home and abroad and have concluded that continuing the war can only mean destruction for the nation and prolongation of bloodshed and cruelty in the world. I cannot bear to see my innocent people suffer any longer."
it was shortly after that statement that Japan surrendered. Some historians still believe that had he not gotten involved the Japan military leaders who were ordering kamikaze attacks wouldn't have surrendered.
It's just murky. It's so hard to say one way or the other whether the use of the bombs is what caused the surrender, and even if it was, was it worth it.