Why did they build atomic bombs?

Answer 1

World War II was a very desperate struggle, and it was considered to be necessary to build the most powerful weapons possible, in order to ensure victory. There was also no way of knowing if the enemy was also working on an atomic bomb (or other terrible weapons), so it was better to be the first one to build it and to be able to win before the enemy came up with their own secret weapon.

There was more to it than this. Fission was discovered in Germany in Dec. 1938 and the Nazis had the first government program to develop a bomb. Although many of their top physicists had been forced out, they still had plenty left including Nobel prize winner Warner Heisenberg, who headed the nuclear program. The U.S. had good reason to fear that Hitler would have the bomb before anyone else. They were far ahead in jet aircraft and rockets, yet never came close to exploding an atomic bomb.