Answer : In August 1939, Einstein wrote a letter to Franklin Roosevelt who was then the President of the U.S.
The main purpose of writing the letter Einstein to Franklin Roosevelt was to alert him about the Nazis. He warn that the Nazis were working on a strong and new weapon that could make an atomic bomb.
Fellow physicist Leo Szilard encouraged Einstein to transmit the letter and supported him in preparing it. Franklin D Roosevelt was the American president.
Einstein wrote in his letter that a single nuclear bomb would make mass destruction. So, this was the main purpose of Einstein’s letter to American President Franklin Roosevelt.