While its original purpose was to prove to the world, and especially to the United States, that the Soviet Union was capable of producing such devices, it also brought a surprising twist to the future testing of nukes. The 'Tsar Bomba,' as it became known, was 10 times more powerful than all the munitions used during World War II. The atomic bomb that was tested at Alamogordo in July seemed to offer a very compelling one.On October 30, 1961, the Soviet Union tested the largest nuclear device ever created. Estimates of likely Allied (and especially American) casualties snowballed, some running into the millions, and planners began searching desperately for alternatives. As intelligence assessments of Japan’s actual defensive dispositions began to build up, however, `Downfall’ came under increasing pressure. It was planned to start in November 1945, and was predicted to run well into the spring of 1946. The latter strategy won the day, as being potentially the least costly to the Allies in the long run, and became`Operation Downfall’. The main debate in early 1945, particularly within the US Joint Chiefs of Staff, was whether that defeat would be best achieved primarily by naval blockade and bombing of infrastructure, or by an invasion of the Japanese home islands. Up until that time, the defeat of Japan had been planned and prosecuted with the conventional means of land, sea and air forces. In one split second, the face of war changed completely. It demonstrated a destructive power never before seen in a man-made device. It was no coincidence that on 16th July, the day before the opening of the Potsdam Conference, the world’s first nuclear bomb was detonated in the desert of New Mexico.

It was this knowledge that informed the contents of the Potsdam Declaration, in particular the statement that failure to accept unconditional surrender would result in “prompt and utter destruction” for Japan. However, the Allies were also aware that whilst the Japanese Emperor Hirohito desired an end to hostilities, and would probably accept the unconditional capitulation demanded, the `hawks’ of the Japanese military and civilian leadership were totally opposed to such a humiliating condition and were ready to fight to the finish – whatever that might look like. Why had the Allied powers considered it necessary to inflict such unprecedented destruction on Japanese civilians in order to bring the war to an end? At the Potsdam Conference (17th July – 2nd August 1945) the Allies formulated their terms for ending the war with Japan, which centred on that country’s acceptance of unconditional surrender, as had been the case with Nazi Germany in May. The following day was declared `Victory over Japan’ or VJ Day, although it was not until 2nd September that the final Japanese surrender was signed, thereby bringing the Second World War to a formal close.

On 14th August it finally accepted the demand for unconditional surrender. On 10th August the Japanese government indicated its readiness to accept defeat, subject to certain conditions. The two atomic explosions had the effects desired by the Allies.

Hydrogen bomb vs atomic bomb