The atomic bomb is the result of the Manhattan Project, which Albert Einstein was actually a part of as well. As to why Truman decided to use the atomic bomb against Japan, the answer is involved by several factors.
Location, Japan is isolated, so any harm done would have "directly" been restrained to Japan.
A show of military might, The atomic bomb was the USA's ace in the hole. The adverse effects of this though is that an arms race for nuclear weapons began after WW2 and resulted in The Cold War.
He wanted to end the war very quickly and have the unconditional surrender of Japan. ;
President Truman used the atomic bomb against Japan to quickly end World War II and minimize American casualties. This decision was influenced by military strategies that estimated high death tolls from a ground invasion, the desire to showcase American power, and the urgency to conclude the conflict. The bombings of Hiroshima and Nagasaki ultimately led to Japan's surrender in August 1945.
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