Why did Einstein write a letter to Franklin Roosevelt?

Einstein was a visionary scientist who knew the power of atomic bomb explosions.

When the Nazis came to power in Germany, the American physicists were in an uproar because of the discovery of nuclear fission in Berlin. They believed that the Germans could take advantage and establish atomic bombs. Therefore, after a lot of urging by one of his colleagues, he wrote a letter to Franklin D. Roosevelt on 2 August 1939 to warn about the effects of such a bomb and the magnitude of their explosions.