Why did Einstein write a letter to Franklin Roosevelt?

Albert Einstein, along with physicist Leó Szilárd, wrote a famous letter to U.S. President Franklin D. Roosevelt in August 1939. The purpose of this letter was to warn the American government about the possibility of Nazi Germany developing extremely powerful bombs using nuclear fission. At the time, scientists in Europe had discovered that uranium could undergo chain reactions, releasing an enormous amount of energy. 

If this process was controlled for weapon making, it could result in a bomb far more destructive than anything known before. Einstein himself was not working on nuclear weapons, but his reputation as a leading scientist gave weight to the warning. 

The letter explained that Germany, with its advanced scientific community, had access to uranium resources in Czechoslovakia and could be preparing to develop nuclear weapons. Szilárd, Edward Teller, and Eugene Wigner drafted the letter, but they asked Einstein to sign it because of his global influence. 

This letter prompted Roosevelt to take the idea seriously. As a result, the U.S. set up the Advisory Committee on Uranium, which later expanded into the Manhattan Project, the program that built the first atomic bombs. In short, Einstein wrote the letter to alert Roosevelt about the urgent danger of Germany creating nuclear weapons and to push the U.S. to act quickly in research and defense.