August 2, 1939 – Physicist Leó Szilárd writes a letter signed by Albert Einstein. It is sent to Franklin D. Roosevelt. It is an attempt to warn the president that Nazi Germany may be working on a fission bomb. The U.S. should begin work immediately on its own nuclear weapons research.
September 1, 1939 – Germany invades Poland. Nation upon nation declare war. World War II begins. Roosevelt takes the warnings of Einstein and Szilárd seriously. He begins research out of which develops the Manhattan Project.