What did they build the atomic bomb for?

Answer 1

Atomic bombs were used by the US against Japan at the end of World War II.

Once nuclear physicists and military leaders became aware of the application of nuclear fission for a weapon, it was decided to build atomic bombs, if only to counteract their development by Germany and the Axis. As things turned out, the Germans developed rockets, but never built a nuclear bomb for use with them. By the time the US had its bomb (July 16, 1945), Germany had been defeated without it, and it was used instead as intimidation against the last struggling war efforts of the Japanese.

Rather than invade Japan, the US dropped two atomic bombs on Japanese cities, and implied that they had many more to use (in fact, they had no more, and could only build about two a month). With Japan's anti-invasion plans rendered useless, the Japanese surrendered six days after the second bomb was dropped (Hiroshima on August 6, and Nagasaki on August 9, 1945).

Answer 2

The Americans made it to end the war and they wanted to make it before Hitler made it.