Why United States built the atomic bomb is it built just in case of japan?

Answer 1

The US started building the atomic bomb because it came to their attention that the Nazi scientists in Germany were already working on it. By the start of 1945, the US did not have a ready atomic bomb. Neither did Germany, although they were much closer to it than the US. Once the Allies began their invasion of Germany, the American and Soviet military hunted down the scientists of the Nazi atomic programme. The US was more successful than the USSR in this (of course, the USSR did not actually have an atomic programme yet). With the cooperation of the German nuclear scientists, the US managed to build a successful bomb, but not before Germany surrendered in May 1945. At this point, relations between the US and the USSR were steadily deteriorating.

So to answer your question:

1. the US did not build the bomb 'IN CASE' of Japan - they never suspected that Japan was trying to build one (but they knew for certain that Germany was building a nuclear bomb). So the nuclear programme BEGAN because they feared the Nazis would build a successful one first.

2. When it became certain that the US would not have to use the bomb on Germany, they were still under immense pressure to complete the project because of the money they had spent on it, and because they feared that they would be in another war with the USSR shortly after the war in Europe had ended.

Finally, by using the bomb on Japan:

1. They could avoid a long and costly invasion of Japan and end the war in the pacific quickly.

2. They could show off its strength to the USSR, which had become a new and powerful threat.