I found this: " Nevertheless, it seems clear that, even without the atomic bombing attacks, air supremacy over Japan could have exerted sufficient pressure to bring about unconditional surrender and obviate the need for invasion.
“Based on a detailed investigation of all the facts, and supported by the testimony of the surviving Japanese leaders involved, it is the Survey's opinion that certainly prior to 31 December 1945, and in all probability prior to 1 November 1945, Japan would have surrendered even if the atomic bombs had not been dropped, even if Russia had not entered the war, and even if no invasion had been planned or contemplated.”
US Strategic Bombing Survey of Japan Summary Report, 1 July 1946, p.26 paras. 7–8."
So was nuking really needed?
This is a great question, which gets asked here fairly often. Not discouraging new responses, but this section of our FAQ may be of some interest to you, in particular these threads:
To briefly summarize the above, there is still controversy over what caused Japan to surrender: the atomic bombings/threat of more bombings; the Allied blockade of Japan and general strategic bombing campaign; the Soviet invasion of Manchuria; internal Japanese politics; and so forth. The answer of course is some combination of the above and other factors, but completely reasonable historians disagree over the influence of each.
One of option was to continue the bombing campaigns and keep a blockade around Japan. This would starve what ever meager resources that were coming by sea to stop coming to Japan. Japan would slowly starve and would not be able to make more war resources it needed. This would also cut off any reinforcements to the Manchuria-China front and vice-versa to the home islands. Or any it could make would be subpar and might not work. With this was their is little loss for the US on paper, but the reality is Japan was not going to allow this without a fight. Now that the US might be closer to the Home Islands it easier for Kamikaze attack planes, suicide boats packed with explosives, and torpedo boats to attack these ships. We can see in Okinawa campaign how devastating this was to the US navy. Now the US navy is even closer to bases and ports from which they can strike from. Bombers would also be at risk from AA and attack fighters as well, though the level of intensity would be variable I couldn’t tell you.
This process would take a long time to do with countless casualties and wasted resources. But this was more of a supplement to the main option of Invading Japan.
The other option which was what US was planning on doing was invading mainland Japan. This would involve the US invading the southern most island of Kyushu and taking the southern portion of that island as a assembly area before invading Honshu the largest island of Japan. This would have been the one of the largest invasion of military history, possibly eclipsing D-Day. The plan was to fool the Japanese into thinking they were going to attack somewhere else, invade Kyushu, then prepare for another invasion on Honshu. However some problems arose, Japan knew the US was going to invade Kyushu and transferred units from mainland Asia to defend this area. Combined with other troops in the area that were reinforced and the fact that they have fortified the area, it would be a tough struggle for the US. The Japanese also adapted a Defense in Depth or Elastic defense system. Which meant that they waited for the US forces to take some land and begin establishing a beachhead, then fire on them when they move inland and inflict major casualties. For the US estimated casualties out of its ~550,000 troops about 50,000-108,000 would be casualties. This is not including the Navy losses from Kamikaze attacks, torpedo boats, or suicide boats as well. Which would also target landing craft and supply ships. And this is just for a small portion of Kyushu not the whole of Kyushu. Imagine the intense fighting for cities like Kyoto and Tokyo. This would be costly invasion as almost every civilian was mobilized to fight the US invasion force with spears, grenades, crude firearms, and whatever else they had. It would be a very costly fight for both sides. US casualties was estimated at a 1 million mark, and it could have been a lot higher. The Japanese were estimated to have an 8 million casualties during the campaign, other estimates went from 10-30 million casualties.
The Atomic Bombs while opening Pandora’s Box and letting other countries have this power as well and give us MAD. The other option of invading the Mainland was too costly for the US and would have lead to some of the highest casualties for the US since the Civil War.