World War Two, a Brief Timeline of Events
May 7, 1945
Germany, under the leadership of Reich President Karl Dönitz, surrenders unconditionally to Allied Forces. A day later, Victory in Europe Day would be declared and the allies would focus on the war in the Pacific.
The Battle of Okinawa
A month before VE day, the allied forces launched an invasion of Okinawa, considered to be one of the last stepping stones in the "island hopping" strategy to reclaim the Pacific Islands and eventually invade and defeat Japan. The invasion would last almost three months - until June 22nd - and be one of the most vicious and bloodiest battles of the war, with casualties on both sides being nearly equal. It would be considered to be a taste of what an invasion of the Japanese mainland would be like as Japan had pledged to "prosecute the war to the bitter end."
Weighing the Options
Throughout the war, the allied forces had been conducting numerous conventional bombings on the Japanese mainland while liberating the islands under Japanese control. Prior to the awareness of nuclear weapons, it was naturally assumed that the defeat of Japan would happen by conventional means. This was formalized in a plan known as Operation Downfall, devised by the US Joint Chiefs of Staff during 1945.
Unbeknownst to most, President Truman had formed the Interim Committee, a secret group whose purpose was to advise the present on practical applications of nuclear power. The first order of business: the use of nuclear weapons against Japan. Their conclusion on June 1st:
Mr. Byrnes recommended, and the Committee agreed, that the Secretary of War should be advised that, while recognizing that the final selection of the target was essentially a military decision, the present view of the Committee was that the bomb should be used against Japan as soon as possible; that it be used on a war plant surrounded by workers’ homes; and that it be used without prior warning.
Note, that the eventual defeat of Japan was certain, it was merely a question of how to bring this defeat about expediently with as few casualties as possible. Regardless of which method they would employ, by early July, the general strategy for dealing with Japan politically had been decided: surrender, or be destroyed
The Potsdam Conference
The war was not being fought by the US alone, and it would be necessary to determine how current and future spoils would be divided, and how the political future of Europe and the Pacific would be designed by the victors. From mid-July to early August, the Soviet Union, United Kingdom, and United States (and other leaders) conferred upon these issues.
On July 26th, the US, UK, and Chinese leaders would issue the Potsdam Declaration, publicly demanding the unconditional surrender of Japan, lest it be completely destroyed. Within a few days, Japan would provide a public statement of ignoring the declaration as unimportant.
On August 6th, 1945, the Enola Gay would drop the "Little Boy" atomic bomb on Hiroshima. Three days later, the Bockscar would drop the "Fat Man" on Nagasaki. Within the week, Emperor Hirhohito publicly would issue a surrender.
Behind the Scenes
The story often told of the Japanese as an implacable enemy that would fight to the death, inflicting enormous casualties on all who would dare violate her soil. Such is their confidence that they refused to accept the destruction of Hiroshima, believing it to be a meteor or other natural disaster. Only after the complete destruction of two cities would she bow to a foreign nation.
While it is common to portray the Japanese as being of a single mind, given the face they presented on the world stage, the fact is, the members of the Emperor's cabinet were divided as to how to proceed. One the one side, some argued for fighting to the death in order to force favorable terms while the other argued a quick surrender to minimize damage to the Japanese people. Both camps were of the same mind on one thing: Japan must, at some point, surrender. Were the atomic bombs the tipping point, then? While it is often taught that Japan surrendered not long after the dropping of the second atomic bomb, overlooked is the fact that the Soviet Union had declared war on Japan at the same time.
Ruskies to the Rescue
The Japanese had long feared the Russian army and considered it an impediment to their goals in Asia and the Pacific. Japan had suffered a shocking loss to Russian forces in 1929 and failed to secure any lasting victories against the Soviet Union prior to and throughout World War II. Ultimately, the Soviets and Japanese would sign a pact of neutrality: a mutually beneficial agreement that would allow both nations to ignore each other and focus their forces on a single front. Even when Germany invaded Russia in 1941, Japan opted not to exploit the diversion and maintained neutrality with respect to the Soviets.
In response to the Potsdam Declaration (which the Soviet Union was not a part of), the Japanese government reached out to the Soviets, hoping to use them as a third party to negotiate better terms for surrender. After the bombing of Hiroshima, Japan reached out again, indicating that this act of destruction was not a deciding factor. Even after the second bombing at Nagasaki, Japan considered the Soviet invasion of Manchuria to be more significant. With enemies on all sides and no party to negotiate on their behalf, Japan was forced to offer unconditional surrender.
Much has been made (for and against) of comments of military advisers regarding the necessity of the bombing, most famously that of Dwight D. Eisenhower. More importantly, though, was that the atomic bombings were ineffective, even by the proponents of the bombing. George C. Marshall, in defense of using nuclear bombs, complained, "We killed 100,000 Japanese in one [bombing] raid in one night, but it didn't mean a thing insofar as actually beating the Japanese." Yet the death tolls of Nagasaki and Hiroshima did not significantly eclipse that number, whose casualty numbers include those that later died of radiation poisoning or other indirect causes. Japan had been weathering conventional bombings for over three years at that point, with around 400,000 individuals killed. There is no reason to believe that nuclear weapons were any more effective, having produced similar results to conventional air raids, when previous raids, causing comparable losses also had no effect.
While the atomic bombing of Nagasaki and Hiroshima had a great effect on the world at large, there is little evidence that supports they were necessary to bring about a timely closure to the war. Japan had already internally acknowledged eventual surrender, putting hope in a settlement negotiated by the Russians. This hope was dashed when the Russians denounced their pact of neutrality and invaded Manchuria. The bombings themselves were not significantly more effective than the conventional bombs that had failed to phase Japan over the course of the previous three years. Had the US continued with conventional air raids, it is clear that Japan would have still surrendered after the Soviet invasion, having no other possible hope for a negotiated surrender.
 "My belief that Japan was already defeated and that dropping the bomb was completely unnecessary." - The White House Years