Why did Einstein write a letter to Franklin Roosevelt ? What was the outcome ? Did it have any impact on America ? ( answer in 120-150 words )

Einstein wrote a letter to the American President, Franklin D. Roosevelt on 2 August 1939 in which he tried to warn the American President about the effects of this bomb and also wanted them to make other bomb that could utilize it's potential of destruction. => According to Einstein, bombing of Hiroshima and Nagasaki was unnecessary. Einstein played an important role in the making of atomic bomb but the Americans developed the atomic bomb in a secret project of their own and dropped it on these two Japanese cities. He was later depressed and felt guilty by the extent of destruction. He also wrote a long letter for the formation of World Government in order to stop the destruction caused by the nuclear weapons.