Answer:
I think the letter was about the first atomic bomb.
Explanation:
''The Einstein–Szilárd letter was a letter written by Leó Szilárd and signed by Albert Einstein that was sent to the United States President Franklin D. Roosevelt on August 2, 1939. ... It prompted action by Roosevelt, which eventually resulted in the Manhattan Project developing the first atomic bombs.''
got the explanation from wikipedia, hope this helps.
