Leo Szilard (pictured) penned a letter, signed by Albert Einstein and addressed to U.S. president Franklin D. Roosevelt, warning that Germany may develop atomic bombs, leading to the establishment of the Manhattan Project.

Leo Szilard was a Hungarian-born physicist, biologist and inventor who made numerous important discoveries in nuclear physics and the biological sciences. He conceived the nuclear chain reaction in 1933, and patented the idea in 1936. In late 1939 he wrote the letter for Albert Einstein's signature that resulted in the Manhattan Project that built the atomic bomb, and then in 1945 wrote the Szilard petition asking president Harry S. Truman to demonstrate the bomb without dropping it on civilians. According to György Marx, he was one of the Hungarian scientists known as The Martians.

The Einstein–Szilard letter was a letter written by Leo Szilard and signed by Albert Einstein on August 2, 1939, that was sent to President of the United States Franklin D. Roosevelt. Written by Szilard in consultation with fellow Hungarian physicists Edward Teller and Eugene Wigner, the letter warned that Germany might develop atomic bombs and suggested that the United States should start its own nuclear program. It prompted action by Roosevelt, which eventually resulted in the Manhattan Project, the development of the first atomic bombs, and the use of these bombs on the cities of Hiroshima and Nagasaki.

Albert Einstein was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence formula E = mc², which arises from special relativity, has been called "the world's most famous equation". He received the 1921 Nobel Prize in Physics for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect.

Franklin Delano Roosevelt, also known as FDR, was the 32nd president of the United States from 1933 until his death in 1945. He is the longest-serving U.S. president, and the only one to have served more than two terms. His first two terms were centered on combating the Great Depression, while his third and fourth saw him shift his focus to America's involvement in World War II.